IB-202-4 3-15-06. Most animals have bilateral symmetry The vast majority of animal species belong to the clade Bilateria –Which consists of animals with.

IB-202-4

3-15-06

• Most animals have bilateral symmetry

• The vast majority of animal species belong to the clade Bilateria– Which consists of animals with bilateral

symmetry and triploblastic development

Developmental Patterns (Deuterostome and

Protostome)

• Based on certain features seen in early development– Many animals can be categorized as having

one of two developmental modes: protostome development or deuterostome development

Cleavage

• In protostome development– Cleavage is spiral and determinate

• In deuterostome development– Cleavage is radial and indeterminate

Figure 32.9a

Protostome development(examples: molluscs, annelids,

arthropods)

Deuterostome development(examples: echinoderms,

chordates)

Eight-cell stage Eight-cell stage

Spiral and determinate Radial and indeterminate

(a) Cleavage. In general, protostomedevelopment begins with spiral, determinate cleavage.Deuterostome development is characterized by radial, indeterminate cleavage. Means at the 8 cell stage youcan separate out a cell and will develop into a complete individual

Coelom Formation• In protostome development

– The splitting of the initially solid masses of mesoderm to form the coelomic cavity is called schizocoelous development

• In deuterostome development– Formation of the body cavity is described as

enterocoelous development. Red represents mesodermal cells.

Figure 32.9b

Archenteron

Blastopore MesodermCoelom

BlastoporeMesoderm

Schizocoelous: solidmasses of mesodermsplit and form coelom

Enterocoelous:folds of archenteronform coelom

Coelom (b) Coelom formation. Coelom formation begins in the gastrula stage. In protostome development, the coelom forms from splits in the mesoderm (schizocoelous development). In deuterostome development, the coelom forms from mesodermal outpocketings of the archenteron (enterocoelous development).

Fate of the Blastopore• In protostome development

– The blastopore becomes the mouth

• In deuterostome development– The blastopore becomes the anus. Note two

layers of mesoderm lining body cavity.

Figure 32.9c

Anus

Anus

Mouth

Mouth

Mouth developsfrom blastopore

Anus developsfrom blastopore

Digestive tube

Flat Worms

Figure 33.9

Flatworms• Members of phylum Platyhelminthes

– Live in marine, freshwater, and damp terrestrial habitats

– Have a gastrovascular cavity and are flattened dorsoventrally (significance=don’t need

a circulatory system as diffusion is adequate to meet nutrient distribution from the gut, gas exchange and waste disposal needs because of short distance from cell to environment

• Although flatworms undergo triploblastic development– They are acoelomates (body cavity solid)

• Flatworms are divided into four classes

Class Turbellaria• Turbellarians

– Are nearly all free-living and mostly marine

Figure 33.9

Planaria• The best-known turbellarians, commonly called

planarians– Have light-sensitive eyespots and centralized nerve nets.

Cephalization-concentration of sensory organs and nerve bodies (ganglia) at the anterior end. Allows them to avoid light, seek food etc.

Pharynx. The mouth is at thetip of a muscular pharynx thatextends from the animal’sventral side. Digestive juicesare spilled onto prey, and thepharynx sucks small pieces offood into the gastrovascularcavity, where digestion continues.

Digestion is completed withinthe cells lining the gastro-vascular cavity, which hasthree branches, each withfine subbranches that pro-vide an extensive surface area.

Undigested wastesare egestedthrough the mouth.

Ganglia. Located at the anterior endof the worm, near the main sourcesof sensory input, is a pair of ganglia,dense clusters of nerve cells.

Ventral nerve cords. Fromthe ganglia, a pair ofventral nerve cords runsthe length of the body.

Gastrovascularcavity

Eyespots

Figure 33.10

Other Flatworms are parasitesMonogenea and Trematoda

• Monogeneans and trematodes– Live as parasites in or on other animals– Parasitize a wide range of hosts

Life cycle of blood fluke (Schistosoma)

• Trematodes that parasitize humans– Spend part of their lives in snail hosts

These larvae penetratethe skin and blood vessels of humans working in irrigated fields contaminated with infected human feces.

Asexual reproduction within a snail results in another type of motilelarva, which escapes from the snail host.

Blood flukes reproduce sexually in the human host. The fertilized eggs break throughblood vessel wall into the digestivetract and exit the host in feces.

The eggs develop in water into ciliated larvae. These larvaeinfect snails, the intermediate hosts.

Snail host

1 mm

Female

Male

5

2

3

4

Figure 33.11

Mature flukes live in the blood vessels of the human intestine. A female fluke fits into a groove running the length of the larger male’s body, as shown in the light micrograph at right.

1

• Most monogeneans– Are parasites of fish

(Cestode) Tapeworm• Tapeworms (highly specialized for parasitic life)

– Lack a digestive system—absorb nutrients from intestinal lumen. Specialized head for attachment to gut wall.

Proglottids withreproductive structures

200 µm

Hooks

SuckerScolex

Rotifers

• Rotifers, phylum Rotifera– Are tiny animals that inhabit fresh water, the

ocean, and damp soil and are of interest because of their prolonged asexual reproduction.

• Rotifers are smaller than many protists (largest 2 mm)– But are truly multicellular and have specialized organ systems

(digestive tract with mouth and anus, body cavity but is pseudocoelom-not lined by mesoderm).

0.1 mmFigure 33.13

Rotifer Reproduction

• Rotifers reproduce by parthenogenesis (unfertilized eggs develop into adult females) which produce more females from unfertilized eggs.

• Without sexual reproduction, deleterious mutations should accumulate that would make the organism less fit. However this has been going on for 35 million years and how they flout the general rule against long-lived asexuality is a puzzle.

Nemerteans

• Members of phylum Nemertea– Are commonly called proboscis worms or

ribbon worms, pseudocoelom)

Figure 33.15

• The nemerteans unique proboscis– Is used for defense and prey capture (some inject toxin into prey)– Is extended by a fluid-filled sac

• Nemerteans also have a closed circulatory system (none of the other phyla we talked about have this).– The blood is contained in vessels distinct from fluid in the body

cavity but there is no heart. Fluid moved through the system by contraction of body wall. (Antarctic nemerteans can capture a 6 inch long fish and ingest it. Cut them in half and regenerate). Mucous contains tetrodotoxin -a nerve toxin).

The Annelids

Segmented worms.

• The phylum Annelida is divided into three classes

Table 33.4

Oligochaetes

• Oligochaetes (class Oligochaeta)

• The earth worms (fish bait)– Are named for their relatively sparse

chaetae, or bristles made of chitin– Include the earthworms and a variety of

aquatic species

• Earthworms eat their way through the soil, extracting nutrients as the soil moves through the alimentary canal– Which helps till the earth, making

earthworms valuable to farmers

• Anatomy of an earthworm

MouthSubpharyngealganglion

Pharynx EsophagusCrop

Gizzard

Intestine

Metanephridium

Ventralvessel

Nervecords

Nephrostome

Intestine

Dorsalvessel

Longitudinalmuscle

Circularmuscle

Epidermis Cuticle

Septum(partitionbetweensegments)

Anus

Each segment is surrounded by longitudinal muscle, which in turn is surrounded by circular muscle. Earthworms coordinate the contraction of these two sets of muscles to move (see Figure 49.25). These muscles work against the noncompressible coelomic fluid, which acts as a hydrostatic skeleton.

Coelom. The coelom of the earthworm is partitioned by septa. AllowIndependent movementof body wall.

Metanephridium. Each segment of the worm contains a pair of excretory tubes, called metanephridia, with ciliated funnels, called nephrostomes. The metanephridia remove wastes from the blood and coelomic fluid through exterior pores.

Tiny blood vessels are abundant in the earthworm’s skin, which functions as its respiratory organ. The blood contains oxygen-carryinghemoglobin.

Ventral nerve cords with segmental ganglia. The nerve cords penetrate the septa and run the length of the animal, as do the digestive tract and longitudinal blood vessels.

The circulatory system, a network of vessels, is closed. The dorsal and ventral vessels are linked by segmental pairs of vessels. The dorsal vessel and five pairs of vessels that circle the esophagus of an earthworm are muscular and pump blood through the circulatory system.

Cerebral ganglia. The earthworm nervous system features a brain-like pair of cerebral ganglia above and

in front of the pharynx. A ring of nerves around the pharynx connects to a subpharyngeal ganglion, from which a fused

pair of nerve cords runs posteriorly.

Chaetae. Each segment has four pairs of

chaetae, bristles that provide traction for

burrowing.

Many of the internal structures are repeated within each segment of

the earthworm.

Giant Australian earthworm

Clitellum

Table 33.23

Polychaetes• Members of class Polychaeta

– Possess paddlelike parapodia that function as gills and aid in locomotion, bristles and gas exchange.

Parapodia

Figure 33.24

• Polychaete Christmas tree worm

• “Feathers” used to trap food particles which are moved along their base to the mouth--also gas exchange.

Figure 33.1

Leeches• Members of class Hirudinea (1 to 30 cm, tropics)

– Are blood-sucking parasites, such as leeches. Secrete hirudin (anticoagulant) and drink 10 x their body weight in blood. Medicinal leech used to drain blood from injured finger.

Figure 33.25

Nematoda (non-segmented round worms)

• Grouped with arthropods in Ecdysozoa clade because covered with cuticle that it sheds as it grows

• The cylindrical bodies of nematodes (phylum Nematoda)– Are covered by a tough coat called a cuticle

25 µmFigure 33.26

• Fluid filled pseudocoelom acts as a hydroskeleton that the circular and longitudinal muscles work against

• Great numbers of nematodes live in moist soil and lake bottoms. Are agriculture pests (corn root worm) and parasites of animals and humans. These include pinworms and round worms in the intestinal tract.

Can acquire this parasite from eating uncooked pork of infected swine. The encysted juveniles mature and migrate from your intestine into the intestinal muscle. Produce more juveniles which migrate to the heart where they encyst forming calcium deposits. Avoid the immune system. Make muscle cell bigger to house them and vascularization. Painful inflammation.

• Trichinosis (Trichinella) a human disease

50 µmEncysted juveniles Muscle tissue

Figure 33.27

Phylum Mollusca• Includes snails and slugs, oysters and clams,

and octopuses and squids

• Most molluscs are marine– Though some inhabit fresh water and some are

terrestrial

• Molluscs are soft-bodied animals– But most are protected by a hard calcium

carbonate shell

• Molluscs have a muscular foot, a visceral mass, and a mantle

Anatomy of Molluscs

• All molluscs have a similar body plan with three main parts– A muscular foot– A visceral mass– A mantle

Generalized Anatomy of a mollusc

Visceral mass

Mantle

Foot

Coelom Intestine

Gonads

Mantlecavity

Anus

Gill

Nervecords Esophagus

Stomach

ShellRadula

Mouth

Mouth

Nephridium. Excretory organs called nephridia remove metabolic wastes from the hemolymph.

Heart. Most molluscs have an open circulatory system. The dorsally located heart pumps circulatory fluid called hemolymph through arteries into sinuses (body spaces). The organs of the mollusc are thus continually bathed in hemolymph.

The long digestive tract is coiled in the visceral mass.

Radula. The mouth region in many mollusc species contains a rasp-like feeding organ called a radula. This belt of backward-curved teeth slides back and forth, scraping and scooping like a backhoe.

The nervous system consists

of a nerve ring around the

esophagus, from which nerve

cords extend.

Figure 33.16

• Most molluscs have separate sexes– With gonads located in the visceral mass

• The life cycle of many molluscs– Includes a ciliated larval stage called a

trochophore

Trochophore Larva

• Other phyla– Go through a distinct larval stage called a

trochophore larva

Figure 32.13a, b

Apical tuftof cilia

Mouth

Anus(a) An ectoproct, a lophophorate (b) Structure of trochophore larva

• There are four major classes of molluscs

Table 33.3

They look very different, live in diverse environments and have different life histories

Chitons• Class Polyplacophora is composed of the chitons

– Oval-shaped marine animals encased in an armor of eight dorsal plates (Firmly attached to rocks in the intertidal region of the ocean). Try and pry off of a rock-home range.

Figure 33.17

Gastropods

• About three-quarters of all living species of molluscs– Belong to class Gastropoda

A land snail wiith protectiveshell

(a)

A sea slug. Nudibranchs, or sea slugs, lost their shell during their evolution. Gills on dorsal surface.

(b)Figure 33.18a, b

• Most gastropods– Are marine, but there are also many

freshwater and terrestrial species– Possess a single, spiraled shell

• Slugs lack a shell– Or have a reduced shell

Torsion in Gastropods• The most distinctive characteristic of this class

– Is a developmental process known as torsion, which causes the animal’s anus and mantle to end up above its head. To accomodate shell?? Some

have flattened shells (abalone). Head with eye stalks. Modified radula as poisonous dart (marine cone snail toxin can kill humans).

Anus

Mantlecavity

StomachIntestine

Mouth

Figure 33.19

Bivalves

• Molluscs of class Bivalvia– Include many species of clams, oysters, mussels, and scallops.

Were used as food sources by early man as well as today.– Have a shell divided into two halves

Figure 33.20

Anatomy of a Bivalve• The mantle cavity of a bivalve

– Contains gills that are used for feeding as well as gas exchange

Hinge areaGut Coelom

Heart

Adductormuscle

AnusExcurrentsiphon

Waterflow

IncurrentsiphonGill

Mantlecavity

Foot

Palp

Mouth

Shell

Mantle

Figure 33.21

Cephalopods

• Class Cephalopoda includes squids and octopuses– Carnivores with beak-like jaws surrounded by tentacles of their

modified foot– Closed circulator system– Well developed eye similar to vertebrate eye (lens, retina etc)– Very active life style. Squid can feed on herring by zipping

through a school. Herring capable of rapid swimming. – Elaborate sex where male inserts packets of sperm into mantle

cavity of female. (Will look at squid in lab).– Ink gland for escape.

• Most octopuses– Creep along the sea floor in search of prey

Figure 33.22a(a) Octopuses are considered among the

most intelligent invertebrates.

• Squids use their siphon– To fire a jet of water, which allows them to swim very

quickly

Figure 33.22b(b) Squids are speedy carnivores with

beaklike jaws and well-developed eyes.

`• One small group of shelled cephalopods– The chambered nautiluses, survives today– Huge nautilus fossils in northern Africa

Figure 33.22c

(c) Chambered nautiluses are the only living cephalopods with an external shell. Have rudimentary eye without lens like a pin hole camera. Shell is chambered and put less dense ammonium chloride in chamber for flotation. Less dense than seawater.