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Comparing InvertebratesThere are some four million different kinds of animals and plants in the world. Four million different solutions to the problems of staying alive.David AttenboroughEdiacaran Period 600 million years ago.First multicellular animals with bilateral symmetry and segmentation

The Ediacara (/idikr/; formerly Vendian) biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms which lived during the Ediacaran Period (ca. 635542 Ma). Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.[note 1] The Ediacara biota radiated in an event called the Avalon Explosion, 575 million years ago,[1][2] after the Earth had thawed from the Cryogenian period's extensive glaciation. The biota largely disappeared contemporaneously with the rapid appearance of biodiversity known as the Cambrian explosion. Most of the currently existing body plans of animals first appeared only in the fossil record of the Cambrian rather than the Ediacaran. For macroorganisms, the Cambrian biota completely replaced the organisms that populated the Ediacaran fossil record.The organisms of the Ediacaran Period first appeared around 600 million years ago and flourished until the cusp of the Cambrian 542 million years ago when the characteristic communities of fossils vanished.

2Spriggina an Ediacaran fossil displaying bilateral symmetry, cephalization and segmentation.

Spriggina was originally interpreted as annelid or arthropod. However, lack of known limbs, and glide reflected isomers instead of true segments, rejects any such classification despite some superficial resemblance.3Ediacaran ocean scene. This was the last period of the Precambrian and led to the Cambrian Period.

The Ediacaran Period (635-540 mya) was first named by Sokolov in 1952, to distinguish a period of evolutionary radiation among soft-bodied forms found in the Ediacara Hills of South Australia. There has been considerable confusion over the usage of 'Ediacaran' and 'Vendian', and their respective time periods; however, the Ediacaran is now recognized as the appropriate term for this period. Although this period is much less well-known than the Cambrian that follows, it harbored a radiation of life forms that was nearly as impressive. The ecologically diverse assemblages were dominated by filter-feeding animals, but the first predators also appear--perhaps selecting for the evolution of animals with hard shells or exoskeletons that radiate in the Cambrian. Some Ediacaran fossils are placed into existing phyla; Kimberella is classified as an early, soft-bodied mollusc, Arkarua may be the earliest echinoderm, and Eoporpita, may be a cnidarian. The affinities of other species like Spriggina, Tribrachidium, Charnia, Dickinsonia, and Pteridinium are still debated; they may belong to an existing phylum or they may belong to an extinct 'stem' phylum that flourished during this period of early animal radiations. Indeed, some of the discoid fossils like Ediacaria and Cyclomedusa may not be animals at all; they may be microbial mats. The Ediacaran was a very important period for animal evolution. Newer fossils from the Doushantuo Formation in China, like Vernanimalcula (the oldest bilaterally-symmetrical animal) and fossilized embryos, are contributing to our knowledge of how, when, and why different animal body plans evolved. 4Cambrian Explosion 542 million years ago. Most major phyla appeared here. A fantastic and sudden appearance of hundreds of different species. In just a few million years animals had evolved complex body plans. They had acquired specialized cells, tissues and organs.

The Cambrian explosion, or Cambrian radiation, was the relatively rapid appearance, around 542 million years ago, of most major animal phyla, as demonstrated in the fossil record.[1][2] This was accompanied by major diversification of other organisms.[note 1] Before about 580 million years ago,[note 2] most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years, the rate of evolution accelerated by an order of magnitude[note 3] and the diversity of life began to resemble that of today.[5] Ancestors of many of the present phyla appeared during this period,[6][7] with the exception of Bryozoa, which made its earliest known appearance in the Lower Ordovician.[8]5Invertebrate cladogram this shows the appearance of evolutionary advances.

Ancestor of all invertebrates is a single celled organism.Multicellularity is first important advance from a single cell.6Specialized cells lead to tissues and tissues working together lead to organs.

Note the lung will have other tissues muscle, blood, fat, connective as well.7Radial symmetry body parts from the centerBilateral symmetry right and left halves

Other radial symmetry creatures, jellyfish, hydra, starfish, sea urchins, sand dollars8Echinoderms have Radial SymmetryStar fish Sea Urchin Sea Hedgehog

Echinoderms include starfish, sea stars, brittle stars, sea9Sand Dollars Brittle Stars

Echinoderms (Phylum Echinodermata from Greek, , echinos "hedgehog" und , derma "skin") are a phylum of marine animals. The adults are recognizable by their (usually five-point) radial symmetry, and include such well-known animals as starfish, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species,[1] making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as birds, fish, mammals, and reptiles). Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives.Aside from the hard-to-classify Arkarua (a Precambrian animal with Echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian period. The word "echinoderm" is made up from Greek (echinderma), "spiny skin", cf. (echnos), "hedgehog; sea-urchin" and (drma), "skin", echinodrmata being the Greek plural form.[2]The echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans. The more notably distinct trait, which most echinoderms have, is their remarkable powers of regeneration of tissue, organs, limbs, and of asexual reproduction, and in some cases, complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. Further, it is held by some scientists that the radiation of echinoderms was responsible for the Mesozoic revolution of marine life.

10The two main forms of cnidarians, polyp and medusa have radial symmetry.Nomuras jellyfish 2 m, 450 lbs. Coral Polyps like anemones

Cnidaria (/nadri/[4] with a silent c) is a phylum containing over 10,000[5] species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both. Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming Cubozoa and Scyphozoa possess balance-sensing statocysts, and some have simple eyes. Not all cnidarians reproduce sexually. Many have complex lifecycles with asexual polyp stages and sexual medusae, but some omit either the polyp or the medusa stage.The biggest Jellyfish is named Nomuras Jellyfish and comes from the family of Lionmane Jellyfish, the biggest cnidarian on earth. Nomuras jellyfish is 2 meters tall in average and weighing around 220kg or 450 pounds. In the last view years, the Jellyfish has become serious problem for Japanese fishers and marine ecosystem because they are attacking fishing boats, destroys fishnets, and poisoned fishes with their toxic.11Cephalization nervous tissue becomes concentrated on one end of an organism. This allows for organisms to respond to the environment in more sophiscated ways.

Cephalization is considered an evolutionary trend,[1] whereby nervous tissue, over many generations, becomes concentrated toward one end of an organism. This process eventually produces a head region with sensory organs.[2]Cephalization is intrinsically connected with a change in symmetry. It accompanied the move to bilateral symmetry made in flatworms, with ocelli and pinnae placed in the head region. In addition to a concentration of sense organs, all animals from annelids on also place the mouth in the head region. This process is also tied to the development of an anterior brain in the chordates from the notochord. A notable exception to the trend of cephalization throughout evolutionary advancement is phylum Echinodermata, which, although having a bilateral ancestor, as evidenced by their embryology, develop into a pentaradial animal with no concentrated neural ganglia or sensory head region. However, some echinoderms have developed bilateral symmetry secondarily.In neuroembryology, neural induction of the ectoderm forms a neural tube which undergoes cephalization to form initially three, then five vesicles as a developing embryo. It is the internalized ectoderm which goes on to become the central nervous system, peripheral nervous system and epidermis

12The three germ layers ectoderm, endoderm and mesoderm

A germ layer, is a primary layer of cells that form during embryogenesis.[1] The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans, (animals more complex than the sponge) produce two or three primary germ layers. Animals with radial symmetry, like cnidarians, produce two germ layers (the ectoderm and endoderm) making them diploblastic. Animals with bilateral symmetry produce a third layer (the mesoderm), between these two layers. making them triploblastic. Germ layers eventually give rise to all of an animals tissues and organs through the process of organogenesis.13Segmentation allows for specialization of different parts and minimal new genetic material as each segment is similar to the next.

Coelom a fluid filled cavity between the digestive tube and the outer body wall. It serves as a buffer between the outer wall and the inner organs.

It also allows for growth of internal organs without distorting the outer wall. It serves as a storage space. For invertebrates with an open circulatory system it serves as a place for circulation to occur. The fluid in the cavity serves as a hydrostatic skeleton for many animals.15

The mouth of the animal developing from a protostome develops from the first developmental opening. This opening is called the blastopore. The mouth of the animal developing from the deuterostome develops from the second opening on the dorsal surface, which the blastosphere becomes it's butt. They also have different developmental fates; the protostome's eight cells have determined fates while the deuterostome's eight cells can each develop separately into full organisms.16Intracellular digestion takes place in the cytoplasm of the cells of the organism. This is found in molluscs, cnidarians and Poriferans.This is a view of the Giant Barrel Sponge the largest member of the phyla porifera (pore bearing). It is a filter feeder.

Intracellular digestion is a form of digestion which takes place within the cytoplasm of the organism. Intracellular digestion takes place in animals without a digestive tract, in which food items are brought into the cell for digestion.Intracellular digestion occurs in many unicellular protozoans, in Pycnogonida, in some molluscs, Cnidaria and Porifera. There is another type of digestion, called extracellular digestion. In amphioxus, digestion is both extracellular and intracellular.

17Extracellular digestion Enzymes catalyse the digestion of the food outside the cells and the molecules are absorbed into the blood or body fluids

Extracellular digestion is a process in which saprobionts feed by secreting enzymes through the cell membrane onto the food. The enzymes catalyse the digestion of the food into molecules small enough to be taken up by passive diffusion, transport or phagocytosis. These nutrients are transferred into the blood or other body fluids. Since digestion occurs outside the cell, it is said to be extracellular. It takes place either in the lumen of the digestive system, in a gastric cavity or other digestive organ, or completely outside the body.Extracellular digestion is a form of digestion found in all saprobiontic annelids, crustaceans, arthropods, lichens and chordates, including vertebrates.

18Cnidarian digestion Flatworm digestionIn both animals there is one opening. So wastes will go out the same tube that the food went in.

19Digestive tract one way flow from mouth to anus. Specialized cells help to digest the food, move the food, absorb nutrients, get rid of wastes, making the whole system more efficient.

The specialized tissues allow the organism to digest more food, more efficiently. 20Porifera respiration. Sponge cells absorb oxygen directly from the water. They eliminate carbon dioxide the same way.

Cnidarian Respiration. There are no respiratory organs. Both cell layers absorb oxygen from the water.

There are no respiratory organs, and both cell layers absorb oxygen from and expel carbon dioxide into the surrounding water. When the water in the digestive cavity becomes stale it must be replaced, and nutrients that have not been absorbed will be expelled with it. Some Anthozoa have ciliated grooves on their tentacles, allowing them to pump water out of and into the digestive cavity without opening the mouth. This improves respiration after feeding and allows these animals, which use the cavity as a hydrostatic skeleton, to control the water pressure in the cavity without expelling undigested food.[6]Cnidaria that carry photosynthetic symbionts may have the opposite problem, an excess of oxygen, which may prove toxic. The animals produce large quantities of antioxidants to neutralize the excess oxygen

22Cnidarian cnidoctye. An explosive cell containing harpoon like cnida which also has a toxin. Some species (portugeese man of war, sea wasp) are dangerous to people.

A cnidocyte is an explosive cell containing one giant secretory organelle or cnida (plural cnidae) that defines the phylum Cnidaria (corals, sea anemones, hydrae, jellyfish, etc.). Cnidae are used for prey capture and defense from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on fish and crustaceans. A cnidocyte fires a structure that contains the toxin, from a characteristic sub-cellular organelle called a cnidocyst (also known as a cnida or nematocyst). This is responsible for the stings delivered by jellyfish.23Box jellyfish (class Cubozoa) are cnidarians distinguished by their cube-shaped medusa. Some species produce extremely potent venom. Three are among the most venomous creatures in the world. Stings from these are extremely painful and sometimes fatal to humans.Box JellyfishA stinging victim

Once a tentacle of the box jellyfish adheres to skin, it pumps nematocysts with venom into the skin, causing the sting and agonizing pain. Domestic vinegars have been confirmed as an effective treatment as they disable the sea wasp's nematocysts not yet discharged into the bloodstream. Pressure immobilisation can also be used on limbs to slow down the spreading of the deadly venom. Common practice is to apply generous amounts of vinegar prior to and after the stinging tentacle is removed.[25] Removal of additional tentacles is usually done with a towel or gloved hand, to prevent secondary stinging. Tentacles will still sting if separated from the bell, or after the creature is dead. Removal of tentacles without prior application of vinegar may cause unfired nematocysts to come into contact with the skin and fire, resulting in a greater degree of envenomation24Portuguese Man of War. This cnidarians nematocysts (cnidocytes) can also penetrate human skin and leave a painful rash.

Stings from a Portuguese man-of-war may result in a severe dermatitis.[14]:429[15] The Portuguese man o' war is often confused with jellyfish, which may lead to improper treatment of stings, as the venom differs from that of true jellyfish. Treatment for a Portuguese man o' war sting includes:avoiding further contact with the Portuguese man o' war and carefully removing remnants of the organism from the skin (taking care not to touch them directly with fingers or any other part of the skin to avoid secondary stinging); thenTo apply salt water to the affected area (not fresh water, which tends to make the affected area worse)[16][17]To follow up with the application of hot water (45C/113F) to the affected area from anywhere between 15-20 minutes [18] which eases the pain of a sting by denaturing the toxins.[19]If eyes have been affected, irrigate with copious amounts of room-temperature tap water for at least 15 minutes, and if vision blurs or the eyes continue to tear, hurt, swell, or show light sensitivity after irrigating, or there is any concern, to seek medical attention as soon as possible.

25Respiratory system. The purpose of this system is to bring oxygen to the cells of the body and to eliminate carbon dioxide from the body. All have a large surface area and all are moist. In the anatomy of a typical mollusc (a gastropod snail) the paired gills can be seen.

The respiratory system (or ventilatory system) is a biological system consisting of specific organs and structures used for the process of respiration in an organism. The respiratory system is involved in the intake and exchange of oxygen and carbon dioxide between an organism and the environment.26Squid Gills. Squid are in the class cephalopoda (head footed) and the phylum mollusca.

Respiration in land invertebrates. Respiratory surfaces are covered with water or mucus. These organs are usually covered to minimize water loss. Sow bugs or wood lice are crustaceans that breathe through paddle shaped hind legs. (pleopodal lungs).

Though today found worldwide, woodlouse populations in the Americas arrived from Europe by sea alongside humans.[19]Living in a terrestrial environment, woodlice breathe through trachea-like lungs in their paddle-shaped hind legs (pleopods), called pleopodal lungs. Woodlice need moisture because they rapidly lose water by excretion and through their cuticle, and so are usually found in damp, dark places, such as under rocks and logs, although one species, Hemilepistus reaumuri, inhabits "the driest habitat conquered by any species of crustacean".[21] They are usually nocturnal and are detritivores, feeding mostly on dead plant matter

28Arachnids. Spiders breathe through specialized structures called book lungs.

Spiders have developed several different respiratory anatomies, based on book lungs, a tracheal system, or both. Mygalomorph and Mesothelae spiders have two pairs of book lungs filled with haemolymph, where openings on the ventral surface of the abdomen allow air to enter and diffuse oxygen. This is also the case for some basal araneomorph spiders like the family Hypochilidae, but the remaining members of this group have just the anterior pair of book lungs intact while the posterior pair of breathing organs are partly or fully modified into tracheae, through which oxygen is diffused into the haemolymph or directly to the tissue and organs.29Insects breathe through a series of openings (spiracles) that lead to tiny vessels (tracheal tubes) that bring oxygen to the cells of the body.

The respiratory system of insects (and many other arthropods) is separate from the circulatory system. It is a complex network of tubes (called a tracheal system) that delivers oxygen-containing air to every cell of the body. Air enters the insect's body through valve-like openings in the exoskeleton. These openings (called spiracles) are located laterally along the thorax and abdomen of most insects -- usually one pair of spiracles per body segment. Air flow is regulated by small muscles that operate one or two flap-like valves within each spiracle -- contracting to close the spiracle, or relaxing to open it. After passing through a spiracle, air enters a longitudinal tracheal trunk, eventually diffusing throughout a complex, branching network of tracheal tubes that subdivides into smaller and smaller diameters and reaches every part of the body. At the end of each tracheal branch, a special cell (the tracheole) provides a thin, moist interface for the exchange of gasses between atmospheric air and a living cell. Oxygen in the tracheal tube first dissolves in the liquid of the tracheole and then diffuses into the cytoplasm of an adjacent cell. At the same time, carbon dioxide, produced as a waste product of cellular respiration, diffuses out of the cell and, eventually, out of the body through the tracheal system.30Open circulatory systems. Blood is only partially contained within a system of vessels. One or more hearts pump blood through vessels into a system of spongy cavities (sinuses). Arthropods and most molluscs.

The cephalopods are exceptions to the other molluscs in that they have a closed circulatory system. Their blood is blue because of the pigment hemocyanin which is different from our pigment (hemoglobin)

The squid has a relatively complex circulatory system for invertebrates, with a closed circulatory system. The squid has two branchial hearts at the base of the gills which send unoxygenated blood through the gills. The third ventricular heart then pumps this oxygenated blood throughout the body.Squid have three hearts. Two branchial hearts feed the gills, each surrounding the larger systemic heart that pumps blood around the body. Squid blood contains the copper-rich protein hemocyanin for transporting oxygen. The faintly greenish hearts are surrounded by the renal sacs - the main excretory system. The kidneys are difficult to identify and stretch from the hearts (located at the posterior side of the ink sac) to the liver. The systemic heart is made of three chambers, a lower ventricle and two upper auricles.32Annelids have a closed circulatory system.

The earthworm has a dual circulatory system in which both the coelomaic fluid and a closed circulatory system carry the food, waste, and respiratory gasses. The closed circulatory system has five main blood vessels: the dorsal (top) vessel, which runs above the digestive tract; the ventral (bottom) vessel, which runs below the digestive tract; the subneural vessel, which runs below the ventral nerve cord; and two lateroneural vessels on either side of the nerve cord.[15] The dorsal vessel moves the blood forward, while the other four longitudinal vessels carry the blood to the rear. In segments six through 11, a pair of aortic arches rings the coelom and acts as hearts, pumping the blood to the ventral vessel that acts as the aorta. The blood consists of ameboid cells and hemoglobin dissolved in the plasma. The second circulatory system derives from the cells of the digestive system that line the coelom. As the digestive cells become full, they release non-living cells of fat into the fluid-filled coelom, where they float freely but can pass through the walls separating each segment, moving food to other parts and assisting in wound healing]33Insects have an open circulatory system. The blood moves from the hearts into the body sinuses and is collected again near the head

Since oxygen is delivered directly, the circulatory system is not used to carry oxygen, and is therefore greatly reduced. The insect circulatory system has no veins or arteries, and instead consists of little more than a single, perforated dorsal tube which pulses peristaltically. Toward the thorax, the dorsal tube (element 14) divides into chambers and acts like the insect's heart. The opposite end of the dorsal tube is like the aorta of the insect circulating the hemolymph, arthropods' fluid analog of blood, inside the body cavity.34Excretory system controls the loss of water and gets rid of nitrogenous waste. (ammonia) Flatworms use a system of flame cells to eliminate excess water. Ammonia diffuses directly through the animals skin.

A flame cell is a specialized excretory cell found in the simplest freshwater invertebrates, including flatworms (except the turbellarian order Acoela), rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, removing waste materials. Bundles of flame cells are called protonephridia.[1]35Annelids like the earthworm change ammonia to urea in the nephridia and excrete it. This process helps to save water.

Some insects and spiders use Malpighian tubules to change the ammonia into uric acid. This is excreted along with the solid waste as a thick paste. This saves the animal a lot of water.

The Malpighian tubule system is a type of excretory and osmoregulatory system found in some Atelocerata (Insects and Myriapoda), arachnids and tardigrades.The system consists of branching tubules extending from the alimentary canal that absorbs solutes, water, and wastes from the surrounding hemolymph. The wastes then are released from the organism in the form of solid nitrogenous compounds. The system is named after Marcello Malpighi, a seventeenth-century anatomist.It is unclear as to whether the Malpighian tubules of arachnids and those of the Uniramia are homologous or the result of convergent evolution.

37Cnidarians are the first phyla with a specialized nervous system. It is composed of a simple nerve net that helps to coordinate motion of the animal.

The more complex the animal the more developed the nervous system. From simple nerve nets there appear ganglia, lumps of nervous tissue, and finally a brain in the head that controls the nervous system.

Cnidarians, flatworms, this roundworm, annelids and certain molluscs have hydrostatic skeletons. These skeletal systems consist of a fluid filled cavity that provides support and a place for muscles to push.

The round worms a numerous and diverse phyla of organisms. Most are free living but some are parasites hookworm, ascaris, elephantiasis, 40Exoskeletons. In arthropods a outer, hard covering composed of the protein chitin. Sun spiders, like this one from the Las Vegas area have an exoskeleton made of chitin

An exoskeleton (from Greek , x "outer" and , skeletos "skeleton"[1]) is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers and cockroaches, and crustaceans such as crabs and lobsters. The shells of the various groups of shelled mollusks, including those of snails, clams, tusk shells, chitons and nautilus, are also exoskeletons.Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton

41Endoskeletons are structural supports located inside the body. Sea stars and other echinoderms have an endoskeleton made of calcified plates.

The sunflower seastar is the largest starfish with a maximum arm span of 3.3 ft.

Pycnopodia helianthoides, commonly known as the sunflower seastar, is a large sea star found in the northeast Pacific. It is the largest sea star in the world, with a maximum armspan of 1m (3.3ft). Sunflower seastars usually have 16 to 24 limbs; their color can vary widely. They are predatory, feeding mostly on sea urchins, clams, snails, and other small invertebrates.Sunflower seastars can grow to have an arm span of 1m (3.3ft) in diameter.[2] Their color ranges from bright orange, yellow and red to brown and sometimes to purple, with soft, velvet-textured bodies and 16 to 24 arms with powerful suckers.[2][3] Most sea star species have a mesh-like skeleton to protect their internal organs.

43

Sexual Reproduction most invertebrates reproduce sexually during at least a part of the life cycle. In sponges sperm and egg combine to form a free swimming larva that settles and develops into a sponge.

Sponges are animals of the phylum Porifera (/prfr/; meaning "pore bearer"). They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. Sponges have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.Most species use sexual reproduction, releasing sperm cells into the water to fertilize ova that in some species are released and in others are retained by the "mother". The fertilized eggs form larvae which swim off in search of places to settle. Sponges are known for regenerating from fragments that are broken off, although this only works if the fragments include the right types of cells. A few species reproduce by budding. When conditions deteriorate, for example as temperatures drop, many freshwater species and a few marine ones produce gemmules, "survival pods" of unspecialized cells that remain dormant until conditions improve and then either form completely new sponges or recolonize the skeletons of their parents.45Life cycle of a jellyfish13 Larva searches for site, 48 Polyp grows911 Polyp strobilates1214 Medusa grows

In the Cnidaria sexual reproduction often involves a complex life cycle with both polyp and medusa stages. For example in Scyphozoa (jellyfish) and Cubozoa (box jellies) a larva swims until it finds a good site, and then becomes a polyp. This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile medusae, a process called strobilation. The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically. The adults have gonads in the gastroderm, and these release ova and sperm into the water in the breeding season46Life cycle of a cnidarian polyp. (Coral, Sea Anemone)

Elkhorn coral at Molasses Reef Florida keys. These hard corals contain thousands of polyps.

Sexual planaria are hermaphrodites each animal possessing both testicles and ovaries. Each planarian gives and receives sperm. Eggs develop inside the body.

Sexual planaria are hermaphrodites, possessing both testicles and ovaries. Thus, one of their gametes will combine with the gamete of another planarian. Each planarian transports its excretion to the other planarian, giving and receiving sperm. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. Sexual reproduction is desirable because it enhances the survival of the species by increasing the level of genetic diversity.49Nematode Sexual reproduction. Most species have a male and a female worm. The eggs may hatch inside the female or they are deposited outside.

Most nematode species are dioecious, with separate male and female individuals. Both sexes possess one or two tubular gonads. In males, the sperm are produced at the end of the gonad, and migrate along its length as they mature. The testes each open into a relatively wide sperm duct and then into a glandular and muscular ejaculatory duct associated with the cloaca. In females, the ovaries each open into an oviduct and then a glandular uterus. The uteri both open into a common vagina, usually located in the middle of the ventral surface.[33]Reproduction is usually sexual. Males are usually smaller than females (often much smaller) and often have a characteristically bent tail for holding the female. During copulation, one or more chitinized spicules move out of the cloaca and are inserted into the genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm. Nematode sperm is thought to be the only eukaryotic cell without the globular protein G-actin.Eggs may be embryonated or unembryonated when passed by the female, meaning their fertilized eggs may not yet be developed. A few species are known to be ovoviviparous. The eggs are protected by an outer shell, secreted by the uterus. In free-living roundworms, the eggs hatch into larvae, which appear essentially identical to the adults, except for an underdeveloped reproductive system; in parasitic roundworms, the life cycle is often much more complicated

50Annelid sexual reproduction. Earthworms are hermaphroditic. Sperm will be exchanged and a cocoon will form around the clitellum were the eggs and sperm will unite. The young worms are like the adults.

Most mature clitellates (the group that includes earthworms and leeches) are full hermaphrodites, although in a few leech species younger adults function as males and become female at maturity. All have well-developed gonads, and all copulate. Earthworms store their partners' sperm in spermathecae ("sperm stores") and then the clitellum produces a cocoon that collects ova from the ovaries and then sperm from the spermathecae. Fertilization and development of earthworm eggs takes place in the cocoon. Leeches' eggs are fertilized in the ovaries, and then transferred to the cocoon. In all clitellates the cocoon also either produces yolk when the eggs are fertilized or nutrients while they are developing. All clitellates hatch as miniature adults rather than larvae51Cephalopods like this cuttlefish are male and female. Fertilization is internal here but can be external with other speices.

Arthropod sexual reproduction. Most have males and females. Some external fertilization, but most internal fertilization. Most lay eggs though scorpions give live birth.

A few arthropods, such as barnacles, are hermaphroditic, that is, each can have the organs of both sexes. However, individuals of most species remain of one sex all their lives.[33] A few species of insects and crustaceans can reproduce by parthenogenesis, for example, without mating, especially if conditions favor a "population explosion". However most arthropods rely on sexual reproduction, and parthenogenetic species often revert to sexual reproduction when conditions become less favorable.[34] Aquatic arthropods may breed by external fertilization, as for example frogs also do, or by internal fertilization, where the ova remain in the female's body and the sperm must somehow be inserted. All known terrestrial arthropods use internal fertilization. Opiliones (harvestmen) and some crustaceans use modified appendages to transfer the sperm directly to the female. However, most male terrestrial arthropods produce spermatophores, waterproof packets of sperm, which the females take into their bodies. A few such species rely on females to find spermatophores that have already been deposited on the ground, but in most cases males only deposit spermatophores when complex courtship rituals look likely to be successful.[33]The nauplius larva of a prawnMost arthropods lay eggs,[33] but scorpions are viviparous: they produce live young after the eggs have hatched inside the mother, and are noted for prolonged maternal care.[35] Newly born arthropods have diverse forms, and insects alone cover the range of extremes. Some hatch as apparently miniature adults (direct development), and in some cases, such as silverfish, the hatchlings do not feed and may be helpless until after their first moult. Many insects hatch as grubs or caterpillars, which do not have segmented limbs or hardened cuticles, and metamorphose into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body.[36] Dragonfly larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws.[37] Crustaceans commonly hatch as tiny nauplius larvae that have only three segments and pairs of appendages

53Echinoderm sexual reproduction. Male and females with the release of eggs and sperm externally and tied to the rhythm of the moon.

Asexual reproduction in sponges. Three ways: by fragmentation, budding and producing gemmules. Gemmules are like spores and are produced by dying sponges

Sponges have three asexual methods of reproduction: after fragmentation; by budding; and by producing gemmules. Fragments of sponges may be detached by currents or waves. They use the mobility of their pinacocytes and choanocytes and reshaping of the mesohyl to re-attach themselves to a suitable surface and then rebuild themselves as small but functional sponges over the course of several days. The same capabilities enable sponges that have been squeezed through a fine cloth to regenerate.[3] A sponge fragment can only regenerate if it contains both collencytes to produce mesohyl and archeocytes to produce all the other cell types.[14] A very few species reproduce by budding.[3]Gemmules are "survival pods" which a few marine sponges and many freshwater species produce by the thousands when dying and which some, mainly freshwater species, regularly produce in autumn. Spongocytes make gemmules by wrapping shells of spongin, often reinforced with spicules, round clusters of archeocytes that are full of nutrients.

55Cnidarian asexual reproduction polyps can reproduce by budding. Hydra commonly reproduces this way.

Flatworm asexual reproduction Planaria will spontaneously pinch off a part of the tail, which will become another planarian

Round worms do not have asexual behavior, but many parasitic ones have very complicated life cycles that involve one or more intermediate hosts.

Annelid Asexual reproduction. Limited to polychetes. Earthworms (oligochetes) can regenerate a lost tail, but do not reproduce asexually. Sabellid worm in parchment tube budding.

Polychaetes can reproduce asexually, by dividing into two or more pieces or by budding off a new individual while the parent remains a complete organism.[6][31] Some oligochaetes, such as Aulophorus furcatus, seem to reproduce entirely asexually, while others reproduce asexually in summer and sexually in autumn. Asexual reproduction in oligochaetes is always by dividing into two or more pieces, rather than by budding.[10][32] However, leeches have never been seen reproducing asexually.[10][33]Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaete genera, Chaetopterus and Dodecaceria, can regenerate from a single segment, and others can regenerate even if their heads are removed.[10][31] Annelids are the most complex animals that can regenerate after such severe damage.[34] On the other hand leeches cannot regenerate

61Arthropod asexual reproduction. Barnacles are hermaphroditic. Some insects (aphids) can reproduce by parthenogenesis. Parthenogenesis involved the growth and development of embryos without fertilization.

A few arthropods, such as barnacles, are hermaphroditic, that is, each can have the organs of both sexes. However, individuals of most species remain of one sex all their lives.[33] A few species of insects and crustaceans can reproduce by parthenogenesis, for example, without mating, especially if conditions favor a "population explosion".Parthenogenesis is a form of asexual reproduction in which growth and development of embryos occur without fertilization. In animals, parthenogenesis means development of an embryo from an unfertilized egg cell and is a component process of apomixis62Aphid parthenogensis

Echinoderms have a great capacity for regeneration. Some species can reproduce by parthenogenesis and a few can reproduce asexually.

Echinoderm regeneration. In this case one arm can regenerate the rest of the body.