BioScience 120 - Invertebrates II

 (mollusks, annelids, arthropods, echinoderms)

I. The coelom: A major divergence in animal evolution

A. Development of a True Coelom (Eucoelomate)

1. Has a lining of mesoderm (peritoneum)

2. Two methods of embryonic formation

a. solid mass of mesoderm forms and splits to form two lateral coelomic cavities (schizocoelous)

b. two pouches (evaginations) from the gut from the mesoderm with a pair of coelomic cavities (enterocoelous)

B. These two methods of coelom formation = the two major lines of evolution

1. Protostome line

a. mouth--first opening to form in the embryo

b. cells division in a spiral pattern

c. schizocoelous

2. Deuterostome line

a. mouth--second opening

b. anus or terminal opening first

c. cells division in a radial pattern

d. enterocoelous

II. Phylum Mollusca - snails, clams and their relatives

A. Protostomes

B. Soft bodied (= mollus)

C. Shell of calcium carbonate for protection

D. Mantle - a tissue folded over the body to make the shell

E. Gastropods - stomach footed

1. Diverse - found in all habitats except the air

2. Primitive forms with a twisted body so that the anus empties on the head

3. Flattened foot which is muscular and slimy

4. Shell usually coiled

5. A few very advanced forms--no shell as an adult, but the larvae with shell (veliger)

6. Radula - a muscular belt-like tongue with teeth that scrape food.

F. Bivalves - two shells, one on each side

1. Foot usually a blade-like extensible structure for burrowing in sand and mud.

2. Some (oysters and muscles) stick--hard substrates like rocks or boats

3. Filter feeders

4. Siphons out of the mud or sand

G. Cephalopods - Ahead foots@

1. Foot = arms or tentacles around the mouth

2. Predators - beak for biting prey

3. Mantle cavity = a contractile chamber for "jet propulsion"

4. Only mollusks with a closed circulatory system

a. constant volume

b. high pressure

c. found in high metabolism organisms

5. Well developed nervous system

1. Eyes

2. Some ability to learn

3. Complex behavior

III. Annelida - ringed worms

A. Segmented - many repeat units - some same; others modified

1. Evolutionary (specialization of parts)

2. Redundancy (serial homology)

B. Closed circulatory system

C. Organ systems well developed

D. Marine, fresh water and terrestrial

E. Example - earthworm

1. Nephridia for excretion

2. Muscles

3. Nervous system

4. Digestive system

5. Reproduction - hermaphroditic

F. Marine worms

G. Leeches

IV. Arthropoda - jointed legs

A. Characteristics

1. Exoskeleton

a. protection

b. muscle attachments

c. restrict water loss on land

d. must shed (molt) to increase in size

2. Jointed appendages

3. Highly specialized segments

4. Compound eyes

5. Division of labor in life cycle

6. Open circulatory system

a. larval stages ecologically different from adults

b. sometimes terminal stage for reproduction only

c. dispersal increased

B. Most successful group of organisms in the history of life on earth (greatest number of species)

C. Chelicerata - spiders and their kin

1. Evolved in the oceans; now mostly terrestrial

a. horseshoe crabs and sea spiders

b. scorpions, spiders, ticks and mites

2. Characteristics

a. four pairs of legs

b. pedipalps for sensation and mating

c. chelicerae - fangs with poison glands

d. two main body divisions--Cephalothorax and Abdomen

e. book gills or book lungs

D. Subphylum Mandibulata - specialized mouth parts

1. Crustacea - crabs, crayfish and relatives

a. hard, yet flexible crust (exoskeleton)

b. great variety of organisms

c. varied number of segments

d. marine, freshwater and terrestrial

2. Insecta

a. characteristics

1. 6 walking legs

2. Dorsal wings

3. Three body parts - head, thorax and abdomen

4. Trachea

b. most numerous organisms by number of species

c. freshwater, terrestrial and aerial

d. complex life cycles

1. Gradual - young to adult

2. Nymphs

3. larva, pupa, adult

e. pollinators

f. insects, lice, fleas

3. Myriapods - centipedes and millipedes

a. heads

b. trunk of many segments each with a pair of legs

V. Deuterostome line of evolution

VI. Echinodermata - starfish and relatives

A. Characteristics

1. Calcium skeleton in the body wall

2. Water-vascular system with tube feet

3. Decentralized nervous system

4. Radial symmetry

a. bilateral larvae

b. adaptation to slow movement - sessile

5. Marine only

B. Diversity

1. Starfish

2. Brittle stars

3. Sea cucumbers

4. Crinoids (sea lilies)

WADSWORTH

Mollusca Arthropoda

8272 - 8275 9272 - 9287 Chelicerates

8277 - 8281 9290 - 9298 Mandibulates

8283 - foot 9330 - 9334

9200 - 9202 9359 - 9362

9215 - 9223 Echinoderamata

Annelida 9389 - 9404

9224 - 9230 9408*

9257 - 9261