PHYLUM MOLLUSCA 



283 



Mouth 



Anus 



5 



AMPHINEURA (CHITON) 



Anus- 



Mouth 1 



Mouth 



PELECYPODA (CLAM) 



Mouth- 



GASTROPODA (SNAIL) 

 SCAPHOPODA (TOOTJH SHELL) CEPHALOPODA (SQUID) 



Figure 171. Modifications in the molluscan body plan as illustrated by representatives of the 

 5 classes. Note how the shell (heavy lines), the foot (stippled), and the digestive tract vary in 

 position in the different groups. (After Animals Without Backbones, by Ralph Buchsbaum. 

 Second edition. Copyright 1948 by University of Chicago Press.) 



ANODONTA-A FRESH- 

 WATER CLAM 



Clams usually lie partly buried in the 

 muddy or sandy bottoms of lakes or streams. 

 They burrow and move from place to place 

 by means of the foot (Fig. 172), which can 

 be extended from the anterior end of the 

 shell. Water, loaded with oxygen and food 

 material, is drawn in through a slitlike 

 opening at the posterior end, called the 

 ventral or incurrent siphon; and excretory 

 substances and feces, along with deoxy- 

 genated water, are carried out through a 

 smaller dorsal or excurrent siphon. 



Shell 



The shell consists of two parts called 

 valves. Concentric ridges called lines of 

 growth appear on the outside of each valve; 

 these represent the intervals of rest between 

 successive periods of growth, the annual 

 lines being more conspicuous. The umbo is 



Figure 170. Facing page, representative mollusks. 

 The lines suggest possible relationships. The fig- 

 ures are not drawn to scale. (Based on a diagram 

 by William J. Clench and Ruth D. Turner, Mu- 

 seum of Comparative Zoology, Harvard University. 

 Made expressly for this book.) 



the first part of the shell to develop and is 

 produced in the late veliger stage; it is 

 usually corroded by the carbonic and humic 

 acids in water. 



The outer epithelium of the mantle se- 

 cretes the shell, which consists of three layers 

 (Fig. 173): (1) an outer, thin, horny layer, 

 the periostracum which serves to protect the 

 underlying layers from the acids in the 

 water and gives the exterior of the shell 

 most of its color; (2) a middle portion of 

 crystals of lime (calcium carbonate) called 

 the prismatic layer; and (3) an inner 

 nacreous layer (mother-of-pearl), which is 

 made up of many horizontal layers of cal- 

 cium carbonate, and produces an iridescent 

 sheen. 



Anatomy and physiology 



The valves of the shell are held together 

 by two large transverse muscles called an- 

 terior and posterior adductors, and a dorsal, 

 ligamentous, elastic tissue hinge (Fig. 172). 

 The two folds of the dorsal wall of the clam 

 which line the valves are called the mantle. 

 The space between the mantle flaps, con- 

 taining the two pairs of gill plates, the foot, 

 and the visceral mass is called the mantle 

 cavity. 



