UNSEGMENTED SCHIZOCOELS 



217 



Peanut worms are exclusively marine creatures that 

 can be found from intertidal areas to extreme depths 

 of about 3 miles. Intertidally, they usually are found 

 in sand or mud, within rock cavities, among kelp 

 holdfasts, or beneath rocks. These trochophore- 

 owners possess separate sexes. 



MOLLUSCA (Mollusks) 



Diagnosis: symmetry bilateral; unsegmented, ex- 

 cept in one order of odd chitons (representatives 

 known from about 2-mile depths off the west coast of 

 Mexico); of diverse forms, amphineurans or chitons 

 are of elliptical outline with a large, flat, ventral foot 

 and a shell of eight, overlapping, dorsal valves or plates 

 that may be covered by the skin, the gastropods are 

 generally snail- or slug-like (shell of one piece or 

 absent), the scaphopods are snail-like but have a 

 tubular shell open at both ends, the pelecypods 

 are clam-like (shell of 2 parts or valves) and the 

 cephalopods are the squids and octopuses (shell 

 internal); mollusks are fresh-water, marine, ter- 

 restrial, and parasitic. 



Unfortunately for the beginning student, the mol- 

 lusks have evolved in so many diverse ways that 

 there is no specific, generalized form to be used as a 

 model. However, one can consider the different 

 groups and hypothesize as to the nature of the an- 

 cestral type. Originally, the mollusks probably were 

 bottom-dwelling, bilateral, worm-like forms. They 

 likely crawled upon their bellies by means of a fleshy, 

 muscular projection, the foot. The front end pos- 

 sessed a slightly differentiated head with various 

 sense organs, a concentration of nerve cell bodies 

 (a ganglion), and a mouth. Such an animal may 

 have become almost too large to sprawl out over the 

 bottom and may have been in danger of extinction. 

 Fortunate chance changes in its heredity could have 

 caused a piling of some of the animal's structures on 

 its back. A second type of hereditary change, favor- 

 ing those organisms that possessed it, may have 

 caused the organisms to secrete a shell over their dor- 

 sal mass or hump. This layer which became capable 

 of secreting a shell is assumed to be a skin fold now 

 known as the mantle. Additional hereditary change 

 perhaps caused the mantle to give rise to a pair of 

 gill structures which hung in a posterior cavity be- 

 tween the hump and shell. Possibly this ancestor had 

 further protection by being able to pull its body into 

 the shell. Zoologists are even less sure of its infernal 



anatomy. As a trochophore animal, one can suggest 

 that it had ventral nerve cords connecting to a ''"brain'' 

 by a nerve ring around the food pipe, or esophagus. It 

 seems fair to assume that a particular kind of ex- 

 cretory organ, nephndium, and an incomplete blood system 

 (i.e., one in which the blood is not completely en- 

 closed in blood vessels) leading to a hemocoel (a body 

 cavity in which blood flows) mainly in the foot, were 

 present. There probably was a true body cavity, the 

 coelom, formed entirely within the mesoderm as that 

 layer grew and extended in larval life. In present-day 

 mollusks the coelom is restricted to a small heart 

 cavity (from which the adult excretory organs drain 

 wastes), to the kidney, and to the sex organ cavity. 

 As for the radula, the distinctive rasping structure of 

 most mollusks that is used in food getting, zoologists 

 cannot say whether or not this was primitive, since 

 one whole class lacks it. 



CLASS AMPHINEURA (Chitons, or Sea Cradles) 



Diagnosis: bilaterally symmetrical and unseg- 

 mented except for one order; body elongate, with a 

 reduced head, radula, enlarged and flat ventral foot 

 (sometimes absent), and shell of eight dorsal parts 

 (valves) (Figure 13.5). 



The chitons, also called sea cradles and coat-of- 

 mail shells, generally feed upon marine algae. The 

 different chiton species usually have specialized 

 habitats. Some are found only under rocks of tide 

 pools. One or more common species may be found in 

 depressions in intertidal rocks. Others live on the 

 surface of rocks exposed to the open ocean. A few are 

 found in open tide pools, and some are subtidal. 



Many chitons have a daytime retreat to which they 

 return after nightly feedings. Most use their radula 

 to scrape a film of microscopic plants, diatoms and 

 algae, from rocks. Others feed on seaweed debris. 



The sexes are separate. Typically, eggs are laid in 

 jelly masses or strings, but some chitons lay single 

 eggs. The eggs hatch to a free-swimming, trocho- 

 phore-like larval stage. The free-swimming stage is 

 lost when the larva settles to the bottom. Then fol- 

 lows a series of changes in form (metamorphoses) 

 terminating in the adult. Some species lack the free- 

 swimming stage and the developing young live under 

 the mantle edge of the mother, perhaps for protection. 



After chitons die, their plates, so-called "butterfly 

 shells," wash up on the beach. These items of curios- 

 ity often are the closest contact many people have 



