Coelomates and segmented worms

Question 1

What are the defining features of molluscs? What advancements do they show from the previous animal groups?

Answer 1

1. Eucoelomate protostomes
2. All organ systems well-developed
   - Gas exchange thru gills or lungs
   - Blood pumped by a heart, open circulation. Blood also acts as a hydraulic skeleton.
   - Excretion by metanephridia
   - Digestive system with a feeding (scraping) organ called radula
   - Large brains and complex sense of organs such as eyes
   - Sexes usually separate

Question 2

What is the basic body plan and its modification seen in the molluscs? What are the respective roles of the radula, mantle cavity, and the foot?

Answer 2

Body consists of:

• Head-foot (containing feeding, sensory, and locomotory organs), food used for crawling, clinging, and swimming. Radula used for feeding, like tongue in other animals
• Visceral mass (containing digestive, circulatory, and reproductive organs)
• Mantle encloses a space called mantle cavity that houses gills or lungs. May secrete a shell that overlies the mantle in some

Question 3

What is torsion?

Answer 3

A 180 twisting of visceral mass, mantle, and mantle cavity during early development

• Positions gills, anus, and openings of excretory and reproductive systems behind the head.
• Places mantle cavity with its sensory organs at the anterior end. Allows clean water to enter from front and makes the animal more sensitive to stimuli coming from direction in which it moves

Question 4

How is the shell modified in different molluscs?

Answer 4

Snails- coiled shell
Mussels- consists of two convex halves
Cuttlefish and squids- reduced internal shell
Octopi-no shell
Chitons- shell with several plates
Tooth shells

Question 5

Commercial importance of molluscs

Answer 5

Oysters- pearls and they remove suspended particles from water, reducing turbidity
Snails and slugs- intermediate hosts of many parasites
All consumed as food

Question 6

What is the main advance that first appears with the annelids?

Answer 6

Body (and coelom) segmented

Question 7

By what features would you characterize an annelid?

Answer 7

1. Segmented body and coelom
2. Digestive system with crop, grinding gizzard, and long intestine
3. Pair of nephridia in each segment for excretion
4. Circulatory system closed and segmentally arranged
5. Respiratory nerve cord w/ paired lateral nerves in each segment
6. Central nerve cord with paired lateral nerves in each segment
7. Gonads repeated in many segments
8. Well developed ability for regeneration

Question 8

What is segmentation and what are its advantages?

Answer 8

Division of body into series of ringlike segments and internally the coelom also divided into series of compartments via partitions (septa)

• Creation of hydrostatic compartments make independent movement of separate segments by muscular contractions possible, increases burrowing efficiency
• Allows repetition of body parts creating redundancy, providing safety factor in case of failure
• Allows specialization in certain segments (tagmatization)