Page 1

Arthropods I: Trilobites, chelicerates, and uniramiansZOO 2040 Biology of Animals Topic 11.1

■  Their abundance and wide ecological distribution makes this the most diverse animal group (3/4 of all known species):• Range from the Japanese spider crab (4 m in leg span) to the 0.1 mm

long follicle mite.• All modes of feeding occur among arthropods, but most are

herbivorous.• Some arthropods are serious disease agents and compete with humans

for food; others are beneficial.• Rich fossil history dating to the late Precambrian.

■  Eucoelomate protostomes with well-developed organ systems.■  Similar to annelids, they have distinct metameres.■  The soft cuticle of the ancestors of arthropods was stiffened by

deposition of protein and inert polysaccharide chitin:• Joints had to provide flexibility and a sequence of molts was necessary

to allow for growth.• Molting required hormonal control.• The hydrostatic skeleton function was lost, the coelom regressed and

open sinuses replaced them.• Motile cilia were lost.

Phylum Arthropoda (“jointed legs”) Alternative Arthropod Phylogenies

Alternative Arthropod Phylogenies Arthropod Exoskeletons■  The primitive exoskeleton pattern is a linear series of similar somites

with jointed appendages:• Many somites may be fused or combined into specialized groups called tagmata.• Appendages are often highly specialized for division of labor.

■  The cuticle is highly protective but is jointed, allowing mobility. Consists of an inner thick procuticle and an outer thin epicuticle:• Procuticle has an exocuticle secreted before a molt and an endocuticle secreted

after molting. Both layers contain chitin bound with protein.• Procuticle is lightweight, flexible, and provides protection against dehydration.• Chitin ranges from 40% of procuticle in insects to 80% in crustaceans.• Ca salts makes the procuticle very hard in lobsters and crabs.

■  Cuticle is laminated and further hardened by tanning, a chemical process:• The cuticle is thin between segments, allowing movement at the joints.• Cuticle also folds inward to line the foregut, hindgut and trachea.• Ecdysis (molting), is the process of shedding the outer covering and growing a

new, larger one.• Typically molt 4-7 times; weight is a limit to body size.

Arthropod Form and Function■ Segmentation and appendages promote efficient locomotion:• Usually each somite bears a pair of jointed appendages.• Segments and appendages are then modified for various adaptive functions.• Limb segments are hollow levers with internal striated muscles.• Appendages may function in sensing, food handling, communication, and

walking or swimming.■ Terrestrial arthropods use an efficient tracheal system that

delivers oxygen directly to cells:• Aquatic arthropods respire by various forms of efficient gills.

■ Highly-developed sensory organs• Eyes vary from simple light sensitive ocelli to a compound mosaic eye.• Other senses include touch, hearing, equilibrium, and chemoreception.

■ Complex behavior patterns• Surpass most other invertebrates in complex and organized activities.• Most behavior is innate or unlearned but some is learned.

■ Use diverse resources because of metamorphosis• Many arthropods have metamorphic changes that result in ecologically different

larval and adult stages.• Larvae and adults eat different foods and occupy different habitat and thus avoid

intraspecific competition.

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Arthropods I: Trilobites, chelicerates, and uniramiansZOO 2040 Biology of Animals Topic 11.1

Subphylum Trilobitamorpha■  Arose before the Cambrian, flourished for >300 Mya,!

became rare by end of Devonian (extinct 275 Mya)■  >15,000 spp. Ranged from 0.5 mm to almost 2 m long.■  ‘Tri-lobed’ body shape due to a pair of longitudinal!

grooves. Body divided into a head, thorax, and pygidium:• The head was a fusion of segments, the thorax varied in the!

number and structure of somites, and the pygidium was fused.• Head had antennae, eyes, mouth, and jointed appendages.• Each body somite (except for the last) bore a pair of biramous!

appendages; one branch (ramus) of each appendage was fringed!and was probably a gill; the other ramus was a walking leg.

■  Their exoskeleton contained chitin strengthened by !calcium carbonate (calcite, CaCO3).

■  Compound eyes of some were very complex, with many !lenses (ommatidia) containing calcite!crystals that focused light onto !photoreceptors.

■  Many could roll up like pill bugs.■  Most were bottom dwellers and!

were probably detritovors or !scavengers; some were pelagic!plankton-feeders.

Orders of TrilobitesAnostida: among the early trilobites, with a basic, clamshell-like appearance!.

Phacopida: trilobites with large compound eyes.!Redlichiida: including the most primitive!orders of trilobites in the lower Cambrian.

Proetida: includes some of the last trilobite!species before the Permian Extinction.

Corynexochida: an often spiny group united!by a shared hypostomal attachment.

Asaphida: all share a ventral median suture,!and most a similar development.

!Lichida: some of the most ornately sculptured!species fall into this group.

Ptychopariida: having the ‘generic trilobite’!body plan, but many weird variations!

Evolutionary Trends in TrilobitesThoracic segments were reduced to as few as 2 or increased to > 60, overall body shape was greatly elongated or rendered transverse (widened) in some species:

elongate form transverse form increased segments reduced segments

Miniaturization: reduction in size is adaptive in numerous!microhabitats of complex marine systems, coupled with a!correlation between smaller size and rapid maturity (early !maturation means smaller size at adulthood). When this!reduction is due to progenesis (arrested development), the!trilobites may also display a reduction in the number of !thoracic segments.

Evolutionary Trends in TrilobitesSpinosity: development of spines is commonly !thought of as primarily a defensive adaptation!(as in Comera, at right), but alternate hypotheses!for their adaptive significance include their !functioning as stabilization structures on a loose,! silty substrate (as in Ampyx, below left), or as! flotation/stabilization structures for slow-!swimming taxa (as in Kettneraspis, below right).

!Atheloptic Morphology:!

secondary reduction and!loss of eyes is thought!

to be a trend among !benthic species living !in deep, poorly-lit or !

aphotic habitats.

Pelagic Morphology: development of !extremely large eyes and elongate,!streamlined body shapes associated!with swimming in the lighted!(photic) part of the water column.

Olenimorph: thin exoskeleton, increased numbers of thoracic segments, and a widened, flat body is associated with benthic habitats with low O2 and high concentrations of sulfur compounds. These may represent the first symbiotic relationships between animals and sulfur-reducing bacteria (the thoracic pleurae may have covered large laterally-extended gills, maximizing O2 absorption and providing a large surface area upon which symbiotic bacteria could live.Effacement (loss of surface detail): may be an adaptation for a burrowing lifestyle, or aid in streamlining swimming species.Pitted Fringe: an expansion of the cephalon into a concave chamber perforated by numerous openings is thought to be an adaptation to stabilize the trilobite during filter-feeding.

Evolutionary Trends in Trilobites Evolution of Trilobites

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Arthropods I: Trilobites, chelicerates, and uniramiansZOO 2040 Biology of Animals Topic 11.1

Subphylum !Chelicerata■ Chelicerates have six!

pairs of appendages,!including:• Chelicerae (clawed!

mouthparts).• Pedipalps.• Four pair of legs.

■ They lack mandibles.■ Only group of arthropods without antennae.■ Most suck liquid food from prey.■ Phylogeny of Chelicerata is controversial, especially

relationships of Pycnogonida.

Class MeristomataSubclass Eurypterida (‘sea scorpions’)■  Largest of all arthropods (3 m long).■  Fossils occur in rocks dating from the

Ordovician to the Permian:• Most common ca. 350-450 Mya.• Became extinct 230 Mya.

■  Resemble both marine horseshoe crabs and terrestrial scorpions.

■  Evolved in freshwater or brackish lagoons; may have been amphibious and among the first ‘land animals’.

■  They were powerful predators.■  Their head and thorax had 6 fused

segments, 6 pairs of appendages (1 pair were large claws), and both simple and compound eyes.

■  The abdomen had 12 segments and a spike-like ‘tail’ (telson).

Eurypterus lacustrisSilurian, New York

Mixopterus, a 1-m eurypterid from the early Devonian, Norway

Class MeristomataSubclass Xiphosurida (horseshoe ‘crabs’)■  A ‘living fossil’, nearly unchanged from ancestors!

in the Pennsylvanian. Five spp. in three genera live!in marine coastal waters in U.S. and SE Asia. Feed!at night on worms and small mollusks.

■  Body segments fused into two regions: an unsegmented,!horseshoe-shaped shield or carapace covers the cephalo- !thorax in front of a broad abdomen. The long, pointed!telson has a ball and socket joint to right the animal:• Cephalothorax has five pairs of walking legs, one pair of chelicerae.• Carapace has two compound and two simple eyes.• Abdomen has six pairs of broad, thin, appendages!

fused in the median line; book gills are exposed on!some of the abdominal appendages.

• Walk on bottom and on shore with their walking legs!and swim with abdominal plates.

■  During mating season, they come to shore!during a very high tide:• Females burrow into sand to lay eggs; males follow to add!

sperm before she covers the eggs (important food for birds).• The young larvae hatch and return to the sea at next very high tide.• Larvae are segmented and resemble trilobites.

Class Pycnogonida (“sea spiders”)■  Occur in all oceans, from littoral to abyssopelagic;!

most common in polar waters.■  About 500 spp.; all are predators:

• The mouth (at the tip of a proboscis), sucks juices from !hydroids, soft corals, bryzoans, and sponges.• Many also have chelicerae and palps to remove and!

manipulate pieces of food.■  Range from a few millimeters to much larger.!

All have small, thin bodies:• Some species duplicate the somites and they !

may have five or six pairs of legs.• Some males may have a subsidiary pair of !

legs (ovigers) to carry developing eggs.• The digestive system sends branches into !

the legs; most gonads are also in the legs.■  Most have four simple eyes.■  There is a simple dorsal heart.■  No excretory or respiratory systems.■  Evolved during Devonian. Evolutionary relationships are uncertain;

some place them nearer the crustaceans, while others consider them to be closer to arachnids.

Class Arachnida■  Over 100,000 spp. in 11 Orders and about 550 families.■  Most are free living and more common in warm, dry regions.■  Arachnids’ bodies consist of a cephalothorax and an abdomen:• The cephalothorax bears a pair of chelicerae, a pair of pedipalps and four

pairs of walking legs. Antenna and mandibles are lacking.• Most are predaceous and have claws, fangs, poison glands, or stingers.• Venoms contain digestive enzymes to liquefy prey tissues; sucking

mouthparts ingest the fluids and soft tissues from bodies of their prey.•  Spiders have spinning glands that produce silks of various types.

■  Scorpions first appeared on land in the Silurian; mites and spiders were terrestrial by the end of the Paleozoic.•  Land existence requires internal fertilization. Many species tend their eggs

and/or protect young (at least temporarily).■  Most are harmless to humans and provide essential insect control:• A very few spiders are venomous and can cause pain or death in humans.

Scorpions are much more serious in this regard.•  Ticks may carry human diseases and mites can be crop pests or injurious to other

animals.

Order Scorpionida: Scorpions■  Originally aquatic, they are the oldest known terrestrial!

arthropods (Silurian) and are the largest arachnids (modern!spp. reach 30 cm, fossil spp. were up to 86 cm). 1,200 spp.

■  More common in tropical and subtropical zones worldwide,!but do occur in temperate areas:• Nocturnal and feed largely on insects and spiders, but also on!

small vertebrates and each other.■  The cephalothorax has the appendages, a pair of medial eyes!

and 2-5 lateral eyes:• Two gracile to massive claw-like pedipalps (less venomous!

species tend to have larger claws). Strong chelicerae tear prey.• Preabdomen has seven segments. Postabdomen is the long,!

slender ‘tail’ of five segments that ends in a stinging apparatus (telson).• Telson has venom that varies from mildly to excruciatingly painful and can be very

dangerous (in some areas, more people are killed by scorpions than by snakes). Family Buthidae contains the most dangerous species.

■  Under the abdomen are comb-like pectines that explore!the ground and help in sex recognition (detect!vibrations and are chemoreceptors):• Males perform a ‘mating dance’ in which they guide the!

female over a spermatophore that is attached to the ground.

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Arthropods I: Trilobites, chelicerates, and uniramiansZOO 2040 Biology of Animals Topic 11.1

Order Pseudoscorpionida (“false scorpions”)■  Resemble true scorpions, but lack the long !

postabdomen and stinger; most only 1-7 mm long.■  Cosmopolitan (about 2,000 spp. in 22 families),!

reaching their greatest densities and diversity in!the tropics and subtropics (29 spp. have been!reported from Michigan).

■  Most are terrestrial and live in leaf litter and soil,!in tree hollows, under stones, and within rock!crevices. Still other species are found in caves and in the nests of insects, birds, and mammals. Some live among seaweeds at the high tide mark. Many ‘hitchhike’ (phoresy) on birds, larger insects (especially flies, beetles, and wasps) and harvestmen.

■  All are predaceous and feed on the body juices of other invertebrates.

■  They have venom glands in their pedipalps.■  Most species manufacture silken nests (silk glands are located in

the chelicerae).■  Most have elaborate courtship rituals in which the male entangles

the female in silk; he mates while she eats the silk off of her body.

(Super)order Pedipalpida■ Eight legs, the first pair of which are not for locomotion (these legs are very

long, and the long, many-segmented tarsi are used as ‘feelers’).■ Chelicerae are small, 2-jointed, and chelate. Pedipalps are six-jointed: • In many species, the last 2 segments are modified to form pincers used in seizing prey,

tearing it apart, then passing it to the chelicerae to complete the maceration.■ Harmless and nocturnal, hiding during the day beneath leaves, rocks, or

logs, usually in damp places, although there are desert species.Order Uropygi (whipscorpions)■ Also called “vinegaroons” (spray acetic acid- !

containing fluid from 2 glands that open on each!side of the anus as a defense. 100 spp.

■ The last abdominal segment bears a whip-like!telson, which is very long with some species.

■ Range from only 2 mm to as much as 6.5 cm in length.Order Amblypygi (tailless whipscorpions)■ Greatly flattened. Abdomen lacks both odoriferous!

glands and a whip.■ About 2 cm long. 60 spp.

Order Solifugae (Solpugida)■  ‘Sun spiders’ (they are diurnal), ‘camel spiders’!

(due to their humped head), or ‘wind scorpions’!(because they ‘run like the wind’). 900 spp.!Mostly found in arid areas. Common in N. Africa,!SE to India, Indonesia, (especially Sulawesi)!and in parts of N. America. 1 to 7 cm.

■  Prosoma divided into a large anterior carapace!with a pair of close-set eyes, and a short posterior !region.

■  Abdomen is large and joins broadly to the prosoma.■  First pair of legs is reduced and functions as a tactile organ; they

run on the remaining 3 pairs.■  Powerful, actively-foraging (not ambush)!

predators on invertebrates (and even on!small vertebrates):• Their chelicerae are enormous, pincer-like,!

and used to kill prey (no venom).• Pedipalps leg-like. Have an adhesive!

gland to aid in prey capture.

Order Opiliones■  Harvestmen or ‘daddy longlegs’ are most!

common in late summer and are often are seen!in fields (this and the scythe-like appearance of !their legs accounts for their popular name). Very !common in tropical regions. About 5,000 spp.

■  Body is 1 to 22 mm long:• Unlike spiders, their abdomen and cephalothorax!

join broadly without a narrow pedicel.• They have two eyes on a median knob.• They have eight extremely thin legs, which may be!

20 times their body length.• They can lose (by self-amputation) most of their legs!

without ill effect.■  Chelicerae are pincerlike and they feed more as scavengers than do

spiders. Eat small insects, mites, spiders, carrion, and plant matter.■  Adults have a pair of glands that secrete an evil-smelling fluid.■  Males have a long protrusible penis, are smaller than females, and

often differ markedly from them:• After a pair mates in the autumn, the female uses her long, protrusible

ovipositor to lay her eggs in a crack in the soil.• Shortly after breeding, the parents die; the eggs hatch the following spring.

Order Araneae: Spiders■ About 50,000 spp.■ Body consists of an unsegmented!

cephalothorax and abdomen !joined by a slender pedicel:• Anterior appendages are a pair of !

chelicerae with terminal fangs (one!chela of each chelicera has been lost).• Four pairs of walking legs end in!

claws.• A pair of pedipalps has basal parts that help them !

handle food. In males, the pedipalps are used for sperm transfer.■ May be millions/ha in old fields. All are predaceous (mostly on

insects and other arthropods), which are killed by injected poison and fangs. A few can kill small vertebrates. A few species have killed humans.• Venom liquefies and digests the tissues; fluid mixture is sucked into

stomach.

Silk■ Spinning silk is a critical ability for spiders and some

other arachnids• Two or three pairs of spinnerets!

contain microscopic tubes that!run to silk glands• A liquid scleroprotein secretion!

hardens as it is extruded from !the spinnerets• High tensile strength: used for !

webs, lining burrows, forming !egg sacs and wrapping prey

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Arthropods I: Trilobites, chelicerates, and uniramiansZOO 2040 Biology of Animals Topic 11.1

Araneae Structure and Function■ Breathe by book lungs and/or tracheae• Book lungs are unique to spiders; parallel air pockets extend into a blood-filled

chamber. Air enters the chamber through a slit in the body wall.• Tracheal system is less extensive than in insects; it carries air directly to tissues

■ Have Malpighian tubules for an excretory system• K+, other solutes and waste molecules are secreted into the tubules• Rectal glands reabsorb the K+and water, leaving wastes and uric acid. This

conserves water and allows the organisms to live in dry environments• Many have coxal glands that are modified nephridia at the base of some legs

■ Most have eight simple eyes, each with a lens, optic rods and a retina. They detect movement and may form images.

■ Sensory setae detect air currents, web vibrations and other stimuli■ A (sometimes elaborate) ‘courtship ritual’ is often required before

the female will allow mating• Before mating, the male stores his sperm in his pedipalps• Inserts the pedipalps into the female genital openings.• Eggs may develop in a cocoon in the web or may be carried by the female• The young hatch in about two weeks or overwinter (spiderlings may molt before

leaving the egg cocoon)

Order Acari (ticks and mites)■  Medically and economically the most!

important arachnids■  Found on all continents. About 42,000!

spp. have been described, many more are!estimated to exist• They are both aquatic (often brilliantly colored) and terrestrial: inhabit deserts,

polar areas and hot springs■  Most mites are less than 1 mm long; ticks may range up to 2 cm■  Acarines have complete fusion of cephalothorax and abdomen with no

sign of external segmentation• Mouthparts are on the tip of the anterior capitulum• The chelicerae to each side help pierce, tear or grip food• Other mouthparts include pedipalps with a fused base, the hypostome, a

rostrum and tectum• Adult mites and ticks have four pairs of legs

■  Acarines may transfer sperm directly or by spermatophores• The egg hatches, releasing a six-legged larva; eight-legged nymphal stages

follow

Some Common Acarines■  House dust and carpet mites• Free-living• Feed on particles of shed skin• Shed exoskeletons may produce allergic!

reactions■  Varroa mites parasitize honeybee trachae!

and are wiping out beekeeping in the U.S.■  Spider mites are one of many important agricultural pest mites that suck out plant

nutrients■  Chiggers are larval Trombicula mites; they !

burrow into the skin, feed on dermal tissues!and cause skin irritation

■  The hair follicle mite Demodex is harmless but!other species cause mange in domestic animals

■  The human itch mite (scabies) causes intense!itching

■  Tick species of Ixodes carry Lyme disease■  Tick species of Dermacentor transmit Rocky !

Mountain spotted fever (a rickettsia)

dust mite

human follicle mite

Order Palpigradi (micro-whipscorpions)■  Small (<3 mm long). 60 spp. Live in tropics and warm !

temperate regions. Long abdomen lacks stinging telson.■  Non-chelate pedipalps. First pair of legs act as antennae■  Live beneath stones or in caves■  Predaceous; biology mostly unknownOrder Schizomida (tailless whipscorpions)■  Single family of tropical arachnids. Live beneath!

wood or stones or in litter.■  Fang-like chelicerae. Pedipalps are vertically oriented!

and used for catching prey. First pair of legs act as!antennae. No eyes.

Order Ricinulei (hooded tickspiders)■  45 species in 1 family Range from 5-10 mm■  Thick, often sculptured cuticle, with an anterior !

hood that can be raised and lowered, covering !or revealing the mouth and chelicerae

■  Chelicerae and pedipalps both have small pincers■  Found in Africa and the Americas. Live in leaf mold!

and caves. Move slowly. Feed on other arthropods.■  Mating is similar to spiders. Like Acari, the!

young are six-legged.

Subphylum Myriapoda (“many legs”)■  Have many pairs of appendages (generally many more than

insects and chelicerates):• Like insects, have a single pair of antennae.• No pedipalps or chelicerae (have mandibles and maxillae for

feeding). ■  Have Malpighian tubules for excretion, but these may have

evolved independently.■  Most breathe using a!

tracheal system.■  Have relatively simple!

eyes, sometimes with!several ommatida.

■  Carboniferous species!were the largest land!invertebrates ever !(Arthropleura was > 2 m).

Class Chilopoda (centipedes)■  Elongated, with flattened bodies of up to 177!

somites. 3,000 spp. A new species (at 1 cm long,!the smallest in the world) was discovered in!New York’s Central Park in 2002:• Each somite, except for the one behind the head!

and the last two, bears a pair of jointed legs.• Appendages of the first body segment form !

poison claws.• The head has one pair of antennae, a pair of !

mandibles and one or two pairs of maxillae.• Eyes on either side of the head consist of !

groups of ocelli. A few have compound eyes.■  Terrestrial, found under logs, bark and stones:

• Carnivorous, feed on earthworms, insects and other arthropods.• Most are harmless to humans but a few large, tropical species are dangerous.

■  Salivary glands empty into the anterior of the straight digestive tract.■  Two pairs of Malpighian tubules empty into the hind intestine.■  Elongated heart has a pair of arteries in each somite; ostia provide return

flow of hemolymph. A pair of spiracles in each somite allows air to diffuse through branched air tubes of the tracheae.

■  Dioecious, with unpaired gonads and paired ducts:• Some lay eggs, others are ovoviviparous.• Young resemble adults.

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Arthropods I: Trilobites, chelicerates, and uniramiansZOO 2040 Biology of Animals Topic 11.1

Class Diplopoda (millipedes)■  Among the earliest terrestrial animals (Silurian).■  About 10,000 spp. All terrestrial. Two subclasses: the basal!

subclass Penicillata (tiny ‘bristle millipedes’) and subclass!Chilognatha, with hardened exoskeletons and two infraclasses:• Pentazinia short-bodied ‘pill millipedes’.• Helminthomorpha ‘worm-like’ millipedes), which contains!

the vast majority of species.■  Most eat decayed plants but a few eat living plant tissue.■  Up to 30 cm with 25 to 100 somites.

• Have two pairs of legs per somite, probably from fusion!of two segments (larvae have only one pair of legs to each somite).!Appendages of the seventh somite are specialized as copulatory organs.

• Head has two clusters of simple eyes and a pair each of antennae,!mandibles and maxillae.

• Each abdominal somite has two pairs of spiracles opening into air !chambers and tracheal air tubes.

■  Less active than centipedes; they walk with a undulating !wave of leg movements rather than wriggling motion:• Most roll into a coil for defense. Some secrete toxic or !

repellant fluids (that contain plant metabolites such as!quinones and, often, HCN) from special repugnatorial glands!on sides of body.

• After copulation, the female lays eggs in a nest and guards!them. Newly hatched millipedes generally have only 3 pairs!of legs, adding legs and body segments with each molt.

Class Pauropoda■  Soft-bodied, small (1-8 mm, most ≤ 2 mm) myriapods. About 500!

spp. in 5 families. ■  Poorly-known. Probably related to diplopods.■  Lack eyes (but have a pair of sense organs that resemble eyes).■  Have branched antennae.■  A tergal plate covers each two segments.■  Lack tracheae, spiracles, and a circulatory system.■  The 12 trunk segments bear 9 pairs of legs, no legs on last 2 segments.■  Live in moist soil, litter, decaying vegetation, or under bark and!

debris and feed on fungi.

Class Symphyla■  Small (2-10 mm) with centipede-like bodies. About 160 spp.■  Soft-bodied with 14 segments; 10 or 12 segments have legs.■  One segment has a pair of spinnerets.■  Long, unbranched antennae.■  Also eyeless, with sensory pits at the base of the antennae.■  Tracheal system connects to a pair of spiracles on the head.■  Live in humus, leaf mold and debris. Some are garden pests.■  Males place a spermatophore at the end of a stalk. Female!

stores the sperm in special pouches; removes and!smears them with sperm.

■  Young hatch with only 6 or 7 pairs of legs.

Phylogeny and Adaptive Radiation■ Classically, annelids and arthropods originated from a line of

segmented coelomate protostomes:• In contrast to annelids, arthropods have pronounced tagmatization by fusion

of somites.• Those with primitive characters have appendages on each somite; derived

forms are specialized.■ Modification of exoskeleton and appendages allowed variation

in feeding and movement:• Divergence led to a protoannelid line with laterally located parapodia.• One or more protoarthropod lines had ventrally located appendages.• However, molecular evidence aligns annelids and arthropods with separate

superphyla.■ Recognition of Ecdysozoa and Lophotrochozoa would mean

that cuticles and metamerism arose independently, which also means that annelids and mollusks have separate origins.

■ ‘Arthropodization’ is a crucial development:• The hardening of the cuticle led to most arthropod features.• The hydrostatic skeleton and a closed circulation were unnecessary.• Jointed appendages are a requisite of the hardened cuticle.

Phylogeny and Adaptive Radiation■  ‘Arthropodization’ may have occurred more than once but there are

basic similarities:• Both uniramians and crustaceans have pairs of antennae, mandibles and

maxillae on the head.• The body may be divided into a head and trunk, or into a head, thorax and

abdomen.■ Crustaceans and uniramians likely differ enough to separate them at

the subphylum level:• In most Crustacea, one or more thoracic segments are fused with the head as a

cephalothorax.• Uniramians are nearly all terrestrial; Crustacea are mostly marine with a few

freshwater species.■ Whether Arthropoda are monophyletic or polyphyletic is also

controversial• Tardigrada and Onychophora may be sister taxa to arthropods. Some rRNA

sequence evidence supports their inclusion in the Arthropoda.• Such data also place millipedes and centipedes as sister group to all other

arthropods.• However, these schemes also place crustaceans and insects in a monophyletic

group—a major revision!■ Together, insects and crustacea compose over 80% of all named animal

species.