Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 45 (p. 912)

1. The basic body plan of a mollusk is a visceral mass with a soft epithelium and a muscular foot, and in the mass is contained digestive, excretory, and reproductive organs. This animal may have a differentiated head at the anterior end. The mantle is outside the visceral mass. This allows the gills to develop from its tissues, and the mantle may secrete the protective shell. In the mantle cavity of aquatic mollusks the water passes through, bringing in oxygen and food and carrying out wastes and gametes. This has developed into the jet propulsion system of the squid and octopus.

2. A radula is a (usually) barbed rasping strip that allows mollusks to scrape nutrients off of hard substrates such as rocks (or aquarium walls). In some mollusks, the radula has been secondarily lost (bivalves), and in others it has been modified into a sharp puncturing organ, through which toxic materials or enzymes can be delivered to immobilize or kill prey (cone shells).

3. The mollusk excretory opening is a funnel-like nephrostome that is lined with cilia that collect fluid from the coelom; a coiled tubule leads to the bladder, which is connected to an excretory pore. This type of excretory system allows for concentration of an excretory product.

4. Trochophores are mollusk larvae with a row of cilia around the middle of the body. A veliger is a second larval stage of marine snails and bivalves with a distinct foot, shell, and mantle.

5. Bivalves disperse as larvae because adults are relatively fixed and immobile.

6. Segmentation allows for greater evolutionary potential and flexibility. Segments can be duplicated or modified into specialized regions of the body.

7. Annelid setae are bristles of chitin that serve as anchors in certain crawling movements and may aid in swimming. Parapodia are feathery outgrowths of the body wall next to the setae in polychaete annelids that function in increased surface area for respiration.

8. Earthworms obtain nutrients by sucking soil into their mouth via contraction of the pharynx; then muscles in the gizzard grind up the organic material and the food moves through a long, straight digestive tract, with undigested material deposited as castings. Earthworm sensory structures sense light, touch, and moisture. Earthworms do not have parapodia. Earthworm reproduction is hermaphroditic. They orient head to tail when mating, held together by a mucous band produced by the clitellum. Each exchanges sperm with the other. After separation the clitellum secretes a cocoon, and as it passes the female openings, it receives the eggs. It then picks up the other worm's sperm from the sperm receptacles. Young worms eventually hatch from the cocoon.

9. The prominent feature of lophophorate animals is the lophophore, a circular or U-shaped ridge around the mouth, with one or two rows of ciliated hollow tentacles. This functions in food collection and as a gas exchange organ. The coelomic cavity is within the lophophore and tentacles.