218 



ECTOPROCTA THROUGH ECHIUROIDEA: 



Figure 13.5 Class Amphineura, chiton types: A, hcbnochiton, rep- 

 resenting chitons with freely visible valves; B, Katherina, with o block 

 leathery tunic partially covering its valves; C, Amicuta (= Crypfochifon), 

 with valves indicated but completely covered by its tunic, up to 1 toot 

 long; D, undersurfoce of a chiton, with part of the foot curled away to 

 show the head and mouth (right) and the gills running along the grooved 

 margin. 



with this group. Occasionally one finds costume 

 jewelry made from chiton plates, but chitons are used 

 much less for this purpose than are other mollusks. 



CLASS SCAPHOPODA (Tusk Shells) 



Diagnosis: bilaterally symmetrical and unseg- 

 mented; body elongate and enclosed in an undivided 

 shell that is open at both ends, with a head area of 

 sensory processes, radula, and reduced conical 

 foot (Figure 13.6). 



The tusk or tooth shells apparently feed upon small 

 marine plants and animals. Around the mouth are 

 delicate tentacles; these are sensory and are used to 

 capture food. In a few forms, food is obtained by 

 directing the sea water to the mouth where organisms 

 are filtered and taken into the digestive tract. These 

 are bottom dwellers that usually are partially buried 

 in sand or mud. Most are deep-water forms, but a 

 few are found just below the lowest tide level. 



Figure 13.6 DenloUum, a tooth or tusk shell; not shown is the addi- 

 tional opening at the apex of the shell; Class Scaphopoda. 



The sexes in Scaphopoda are separate. The life 

 history following the laying of single eggs is similar 

 to that of chitons. 



American Indians prized a particular tooth shell, 

 Dentalmm, as money or wampum. Although the In- 

 dians obtained some shells that were washed into 

 shore, their primary means of obtaining tooth shells 

 was by dredging with long rakes from canoes. 



CLASS GASTROPODA (Univalves) 



Diagnosis: body typically asymmetrical, unseg- 

 mented; head well developed, with sense organs; 

 radula and a flat, enlarged ventral foot are present; 

 shell undivided, typically external, internal or absent 

 in some (Figure 13.7). 



The gastropods are fresh-water, land, and marine 

 mollusks that possess a radula. Although the radula 

 is a feeding organ, the food habits of gastropods are 

 variable. Most fresh-water snails are vegetarians, 

 feeding on algal scum on submerged objects, but 



