1

LAB 06: THE LOWER INVERTEBRATES

PHYLUM PORIFERA (sponges)

General Description

Sponges (Fig. 33.4, p. 642) are relatively simple animals. They are said to be at the cellular level of organization because their bodies are not composed of true tissues or organs. Instead they are little more than a colony of several distinctive cell types the serve different functions. One of the distinctive cell types is the choanocyte or collar cell. This cell type lines the internal chambers of sponges. Its flagellum creates the water current that the sponge uses to filter very tiny food particles from the water. The collar that surrounds the flagellum acts like a fine sieve, filtering about bacteria-size food particles. Thus the choanocyte creates the filtering current and captures the food particles which are ingested via phagocytosis. Another cell type found in some, but not all, sponges is the pinacocyte which is a flat cell that forms a thin external covering. Porocytes, found only in the simplest of sponges, are donut-shaped cells that form the incurrent pores called ostia. Archaeocytes are ameboid cells that move about through the body of the sponge and carry out various functions. Some archaeocytes differentiate into the cells responsible for secreting the skeletal elements. These skeletal elements may be mineralized spicules (either calcareous or siliceous) and/or spongin fibers. In addition, some cells secrete collagen fibrils in the intercellular matrix. Sponges are filter feeders because they create a current of water and filter out suspended food particles, typically bacteria and tiny phytoplankton. Water enters the body through the tiny ostia and leaves through the larger oscula. Most sponges are marine. However a few are freshwater.

Phylum Characteristics

1. multicellular, but without true tissues and organs; body is little more than a colony of distinctive cell types; mesohyl separates the two layers of cells

2. either asymmetric or radially-symmetric, benthic filter feeders 3. no digestive tract, rather body consists of choanocyte-lined chambers connected to the exterior via

various pores and/or chambers; digestion is intracellular 4. skeleton involves an extracellular matrix of fibrillar collagen with mineralized spicules and/or

spongin fibers 5. exhibit both asexual (gemmules) and sexual reproduction (usually hermaphroditic)

PHYLUM CNIDARIA (Hydra, Hydroids, Sea Anemones, Corals, Sea Jellies)

General Description

The cnidarians (Fig. 33.7, p. 644) exhibit a higher level of organization than the sponges in their possession of true tissues. Since they do not really possess organs (a few exceptions), the cnidarians are said to have a tissue level of organization. They are constructed two basic tissue layers, an gastrodermis that forms the inner lining of the gut and an epidermis that forms the outer covering of the animal (Fig. 33.5, p. 643). Between these two layers may be found a jelly-like mesoglea which contains wandering amebocytes in some cnidarians. Two radially symmetric body forms occur, the polyp, which is typically elongate along the oral-aboral axis and lives attached to the substrate at the aboral side, and the umbrella-shaped medusa, which is shortened along the oral-aboral axis and lives suspended in the water column as a component of the plankton. In the typical cnidarian life cycle (except the anthozoans, see below), the polyp produces more polyps and/or medusae via asexual budding, while the medusa, as the sexual stage, produces eggs and sperm that combine together as zygotes (Fig. 33.8, p 645). Tentacles surround the mouth in both the polyp and medusa forms. These tentacles, armed distinctive stinging capsules called nematocysts (Fig. 33.6, p. 643) generally capture live prey (although some cnidarians are suspension feeders). Many cnidarians exhibit a symbiotic association with single-celled algae. These algae produce food for their animal hosts through photosynthesis. They may also use the wastes of animal host metabolism.

Windward Community College BIOLOGY 172L Dave Krupp

2

Phylum Characteristics 1. radial symmetry with oral and aboral ends 2. tissue level of organization 3. body composed of two basic tissue layers (i.e., diploblastic): epidermis (= ectoderm) and

gastrodermis (= endoderm with jelly-like mesoglea between; in some groups amoeboid cells wander through mesoglea

4. mouth and digestive cavity (gastrovascular cavity) present, gut is sac-like with no anus (blind-sac gut); can like animals to being a sac-like stomach with tentacles

5. specialized cells (cnidocytes) produce stinging organelles called cnidocysts or nematocysts (Fig. 33.6, p 643)

6. typically possess free-swimming form called a medusa and a sessile attached form called a polyp; characteristically (except in Hydra, sea anemones and corals) exhibit an alternation of generations between polyp (asexual) and medusa (sexual) forms

7. often form colonies of individuals which may be specialized for certain functions (polymorphism), e.g. feeding polyps, defensive polyps and reproductive polyps all within the same colony

8. nervous system is composed of a simple nerve net with simple sensory structures in some 9. hydrostatic skeleton of gastrovascular cavity gives shape and support to the body; some groups

with exoskeletons (hydroids & corals), while some others with endoskeletons (soft corals) 10. typically voracious predators on smaller animal prey 11. most marine, but some freshwater forms exist

Cnidarian Systematics (Table 33.1, p. 644)

Class Hydrozoa (hydroids; Figs. 33.7a, p. 644, & 33.8, p. 645)

1. typically both polyp and medusa forms present; however, polyp usually (but not always) dominant, most often forming colonies of polyps through asexual budding; in many groups, the medusa fails to detach from the polyp to become free-swimming

2. radial symmetry modified as tetramerous symmetry 3. no mesenteries in polyps 4. medusa form, when present, with a velum 5. sexual reproduction involves gamete production by medusae or medusae buds (except in

Hydra) 6. gonads form in the ectoderm of medusa, therefore consider gonads to be external 7. most colonies exhibit polymorphism in the polyp types present 8. mostly marine with some freshwater forms 9. examples: Hydra, a solitary form lacking a medusa stage (all others are colonial); Obelia, a

typical colonial hydroid; Millipora, fire coral (not a true coral); Physalia, the Portuguese man-of-war, is actually a floating colony of tiny polyps

Class Scyphozoa (conventional jellyfish; Fig. 33.7b, p. 644) 1. typically both polyp and medusa forms present; however, medusa form dominant, while polyp

form is reduced or absent 2. polyp, called a scyphistoma, undergoes strobilization to produce ephyra which are immature

medusa stages 3. solitary 4. medusa lacks a velum, but bell margin is scalloped with a sense organ (rhopalium with

statocyst and ocellus) at each notch 5. sexual reproduction involves gamete production by free-swimming medusa 6. gonads form in the endoderm of medusa, therefore consider gonads to be internal 7. all marine 8. examples: Aurelia, the moon jelly; Cassiopeia, the upside-down jellyfish

Class Cubozoa (box jellyfish & sea wasps; Fig. 33.7c, p. 644) 1. polyp and medusa forms present; however, medusa form dominant and polyp form reduced 2. polyp transforms directly into medusa stage without strobilization 3. solitary 4. medusa without a velum, but with a velarium 5. medusa square in cross section with tentacle or group of tentacles at each corner

Windward Community College BIOLOGY 172L 3

6. medusa = sexual generation; gonads endodermal 7. all marine 8. example: Carybdea, a sea wasp

Class Anthozoa (sea anemones, soft corals and stony corals; Figs. 33.7d, p. 644) 1. polyp forms only, medusa stage absent 2. gastrovascular cavity divided by flat walls (mesenteries) that radiate inwards from the body wall 3. internal gonads develop on these mesenteries 4. tubular pharynx connects the mouth to the gut 5. symmetry either hexamerous or octamerous 6. solitary or colonial 7. all marine 8. example: sea anemones, stony corals, soft corals, sea whips, and sea pens, gorgonians, and

precious black corals

PHYLUM PLATYHELMINTHES (flatworms, flukes, and tapeworms)

General Description

Bilateral symmetry tends to be associated with an actively moving life style. Thus bilateral animals have distinctive head (anterior) and tail (posterior) ends. The head end, which is the end that usually enters a new environment first, typically possesses the mouth, prey capture structures, well-developed sense organs, and elaboration of the nervous elements involved with interpreting and integrating sensory information from these sense organs. This development of a head end is called cephalization. In addition to developing distinct head and tail ends, bilaterally symmetric animals tend to develop distinct back (dorsal) and belly (ventral) surfaces. The dorsal surface may often be adapted to protect the top of the animal from potential predators (but not in the flatworms). Thus the dorsal surface may thickened or covered with protective plates. It may also be camouflaged to reduce the visibility of the animal. The ventral surface, being less subject to attack by predators (especially in a crawling animal), may not be as protected. Instead, the ventral surface may be equipped with structures to facilitate locomotion. Acoelomate animals, which include Phylum Platyhelminthes (Fig. 33.9, p. 646), are those animals that lack a fluid-filled body cavity, a coelom. Solid mesoderm fills the space between the ectoderm that lines the exterior of the body wall and the endoderm that forms the innermost lining of the gut. This lack of a coelom impacts movement and circulation since a coelom may act as a hydrostatic skeleton and a circulatory mechanism. Acoelomate animals move either by the action of cilia or muscular movements. Without a coelom or a formal circulatory system, the animal must be small or flat in order to sufficient surface area for diffusion of metabolites across the body wall. Thus flatworms exhibit several advancements over the cnidarians: they are bilaterally symmetric, have a triploblastic body construction, exhibit cephalization and centralizing of the nervous system, and an actively crawling life style. However, their primitive nature is revealed by their acoelomate condition, blind sac gut, and lack of many formal organ systems. Most are free-living in either marine or freshwater habitats. Others are parasitic (see comments on parasitism below).

Phylum Characteristics (Fig. 33.10, p. 647)

1. bilaterally symmetric 2. body dorsoventrally flattened 3. blind-sac gut (gastrovascular cavity) 4. three tissue layers present (triploblastic): ectoderm, endoderm, and mesoderm (technically not a

true mesoderm but we will think of it as such because it is a true tissue layer) 5. acoelomate (lacking a fluid-filled body cavity, a coelom) 6. some cephalization; brain rudimentary, not much more than an aggregation of nerve cells at the

anterior end; nervous system centralized into ladder-like arrangement of nerves; light-sensitive eye spots in some

Windward Community College BIOLOGY 172L Dave Krupp

4

7. possess simple excretory system (flame bulbs - protonephridia) and reproductive system, but no respiratory, circulatory, nor skeletal systems are present

8. free-living forms are marine, freshwater, or terrestrial (moist habitats); parasites may be either endoparasites (internal parasites) or ectoparasites (external parasites)

Systematics

(Table 33.2, p. 646)

Class Turbellaria (free-living flatworms) 1. epidermis is ciliated, often equipped with rhabdites (sticky capsules hypothesized to be related

to cnidarian nematocysts, used in prey capture) 2. mouth usually located centrally on the ventral surface 3. blind-sac gut may be simple or highly branched 4. locomotion involves combination of muscular movements and ciliary action 5. free-living flatworms occupying marine, freshwater, and moist terrestrial habitats 6. example: Dugesia, a planarian

Class Monogenea (monogenetic flukes) 1. typically ectoparasites on skin or gills of fishes and amphibians 2. epidermis (tegument) is syncytial and lacks cilia 3. possess an opisthaptor, a posterior attachment structure bearing suckers and/or hooks 4. mouth is ventrally but positioned anteriorly; digestive system typically Y-shaped 5. life cycle is simple usually involving only one host; ciliated larval stage is free-living 6. example: Gyrodactylus

Class Trematoda (digenetic flukes) 1. typically endoparasites with complex life cycles involving several hosts (Fig. 33.11, p. 647) 2. syncytial tegument 3. possess oral and ventral suckers 4. mouth terminal at the anterior end; digestive system Y-shaped 5. example: human liver fluke

Class Cestoda (tapeworms; Fig. 33.12, p. 648) 1. typically endoparasites with two or more hosts during their life cycles 2. syncytial tegument 3. anterior end modified as a scolex with suckers and hooks 4. no digestive system 5. body divided into units called proglottids, each of which contains both male and female reproductive

organs 6. example: beef tapeworm

Parasitism in the Platyhelminthes

Free-living flatworms possess all of the typical flatworm features and are best illustrated by examples such as the planarian. Parasitic flatworms exhibit a variety of adaptations and specializations for parasitic life styles. Flukes retain many typical flatworm characteristics, but possess structures that enable them to attach to host tissues. Ectoparasitic flukes attach to thei hosts with a structure called an opisthaptor, which is typically armed with hooks and/or suckers. Endoparasitic flukes live in the digestive tracts of their hosts and attach to the gut wall with an oral sucker and a ventral sucker (they do not have an opisthaptor). Tapeworms (Fig. 33.12, p. 648) are endoparasites that attach to the gut walls of their host via a head-like structure called a scolex. Tapeworms are so specialized for a parasitic way of life that they lack a gut. They merely absorb nutrients across their body walls (the tegument). The tegument surface are is greatly increased by the presence of tiny surface projections called microtriches. Tapeworms also reproduce asexually by budding off segments called proglottids behind the scolex. Each proglottid develops a complete male and female reproductive system within it. Mature proglottids are nothing more than bags of eggs. Both flukes and tapeworms exhibit complicated life cycles in which the various stages occupy different specific hosts (e.g., see Fig. 33.11, p. 647).

Windward Community College BIOLOGY 172L 5

Procedure and Assignment

EXAMINATION OF SPONGES 1. Examine the sponge specimens on display. Note the variety of sponge types and forms. 2. Examine the sponge model illustrating a single choanocyte chamber. Note the location of the

choanocytes. Can you recognize other cell types? 3. Examine the commercially-prepared sponge skeleton (w.m.) slide. Compare it to the whole bath

sponges provided. Draw a diagram of the spongin fibers observed under the microscope. 4. Examine the commercially-prepared Grantia spicules slide. Draw a diagram of these spicules. CNIDARIAN ANATOMY AND DIVERSITY 1. Examine the model of Hydra illustrating its anatomy and morphology. Draw a simplified diagram

that labels the following features: body column, mouth, tentacles, epidermis, gastrodermis, gastrovascular cavity, ovary, testis, and asexual bud.

2. Examine the slide labeled Hydra nematocysts. Identify some nematocysts and draw them. 3. Examine the Obelia hydroid colony w.m. slide preparation to identify both feeding and reproductive

polyps. Then examine the Obelia medusa w.m. slide preparation. From your observations draw a detailed labeled diagram that illustrates the life cycle of Obelia.

4. Examine a preserved specimen of Gonionemus in a small dish of water under the dissecting

microscope. Draw a labelled diagram of this animal that illustrates the following features: mouth, manubrium, tentacles, and gonads.

5. Examine the following slides illustrating different stages in the life cycle of the moon jelly Aurelia:

planula, scyphistoma, strobila, and ephyra. Also examine a preserved specimen of an adult moon jelly, noting the following features: oral armrs, tentacles, rhopalia, gonads, and radial canals (drawing a labeled diagram would be useful). Draw a diagram that illustrates all of these stages as components in the life cycle of Aurelia.

6. Cut open one of the preserved sea anemones and identify the following features if present: mouth,

pharynx, tentacles, gastrovascular cavity, mesenteries, and acontia. Also examine the comercially-prepared Metridium (a small sea anemone species) slide. This slide illustrates both transverse and longitudinal section through the anemone. It would be useful to sketch diagrams that illustrate the anatomy of a typical sea anemone. What are the functions of the mesenteries? Compare your observations to the sea anemone and Hydra models.

FLATWORM ANATOMY AND DIVERSITY 1. Examine the commercially-prepared slide illustrating stained whole mounts of planaria (Planaria

Plain and Digestive Tract). Note the eye spots. Are these eyespots image-forming eyes? Also note the branched digestive tract. What type of gut do flatworms have?

2. Examine the commercially-prepared slide of the Chinese liver fluke, Clonorchis sinensis. Identify

the following structures: oral sucker, ventral sucker, gut, excretory pore, testis, ovary, yolk gland, uterus, and seminal vesicle.

3. Examine the commercially-prepared slide of the human blood fluke, Schistosoma mansoni. Note

the relationship between the male and female worms.

Windward Community College BIOLOGY 172L Dave Krupp

6

4. Examine the commercially-prepared slide illustrating different regions of the dog tapeworm, Diplylidium caninum. Indetify the following structures: scolex, mature proglottid, gravid proglottid, testis, ovary, uterus, vas deferens, vitelline gland, vagina, and genital pore.

5. Examine the slides that present different stages of fluke life cycles: sporocyst, rediae, and

cercariae. Note how these stages fit into the life cycle of the Chinese liver fluke, Clonorchis sinensis.

TEXT PAGES COVERED pp. 642 - 648 VOCABULARY ostium/ostia osculum/oscula choanocyte pinacocyte porocyte archaeocyte spicule calcareous siliceous collagen spongin gemmule medusa polyp oral/aboral nematocyst cnidocyst cnidocyte nerve net diploblastic radial symmetry blind-sac gut mesentery gastrodermis epidermis mesoglea ectoderm endoderm planula bud polymorphism zooxanthellae scyohistoma strobilization ephyra velum tetramerous hexamerous octamerous pharynx acoelomate bilateral cephalization triploblastic tegument scolex proglottid rhabdite endoparasite ectoparasite STUDY QUESTIONS 1. Describe filter feeding in sponges. Be sure to mention the path of water through the sponge, the

type food, and the role of the choanocytes. 2. List and describe the skeletal components of sponges. How would you distinguish between

calcareous and siliceous spicules. 3. List and describe the functions of the various cell types of sponges.

4. Diagram a typical cnidarian life cycle such as that exhibited by Obelia. Be sure to indicate the

following stages: polyp, medusa, sperm, egg, zygote, planula. Also point out where asexual reproduction, sexual reproduction, and growth occurs in this cycle.

5. Describe, with the aid of labeled diagrams, the structure and discharging of nematocysts. What

kinds of stimuli are necessary for this discharge? What is the mechanism for discharge? 6. Describe polymorphism as it occurs in the Portugese man-of-war. In your answer, list and define

the function of each type of individual within the colony. 7. Compare and contrast flatworms to cnidarians in the following features: symmetry, gut plan, tissue

layers, grade of organization, cephalization, nervous system, sense organs, excertion and water balance.

8. Describe the adaptations for a parasitic way of life exhibited by ecto- and endo parasitic flatworms.

Be sure to give examples and compare them to free-living forms.

Windward Community College BIOLOGY 172L 7

SYSTEMATICS TO KNOW Phylum Porifera Phylum Cnidaria Class Hydrozoa Class Scyphozoa Class Cubozoa Class Anthozoa Phylum Platyhelminthes Class Turbellaria Class Monogenea Class Trematoda Class Cestoda

Windward Community College BIOLOGY 172L Dave Krupp

8

Figure 1. Basic sponge anatomy (asconoid type) showing cell types

Figure 2. Different sponge body plans : (A) asconoid, (B) leuconoid, and (C) syconoid.

Windward Community College BIOLOGY 172L 9

Figure 3. Close-up view of choanocyte-lined chambers in a leuconoid sponge.

Figure 4. Diagrammatic views of typical cnidarian polyp and medusa body form.

Figure 5. Close-up view of the body wall of Hydra showing cell types.

Windward Community College BIOLOGY 172L Dave Krupp

10

Figure 6. Cnidarian nematocyst discharge.

Figure 7. Sea anemone anatomy, cut-away view revealing mesenteris and gastrovascular cavity.

Figure 8. Medusa types: left, hydrozoan medusa, Gonionemus; right, scyphozoan medusa, Aurelia.

Windward Community College BIOLOGY 172L 11

Figure 9. Hydrozoan (Obelia) lilfe cycle.

Figure 10. Scyphozoan (Aurelia) life cycle.

Windward Community College BIOLOGY 172L Dave Krupp

12

Figure 11. Turbellarian flatworm anatomy.

Figure 12. Transverse section of a turbellarian flatworm through the pharyngeal region.

Windward Community College BIOLOGY 172L 13

Figure 13, Monogenetic trematode.

Liver Fluke (Fasciola)

Liver Fluke Life History

Ova of Parasitic Worms

Schistosoma Mansoni-Human Blood Fluke

Tapeworm (Taenia Pisiformis)