PHYLUM MOLLUSCA 



665 



the valves the dorsal integument (mantle) of Chiton and its 

 Hies is usually beset with a number of horny or calcified tubercles 



mo 



manl- 



5. 562. Chiton spinosus, dorsal view. 

 (From the Cambridge Natural History.) 



FIG. 563. Chiton, ventral view. an. anus ; 

 cten. ctenidia ; ft. foot ; mant. mantle edge ; 

 mo. mouth. (After Pelseneer.) 



and spicules. The mantle develops only very slight lateral flaps, 

 and under cover of these are a series of small ctenidia (Figs. 

 563 and 569, den) to the number of fourteen to eighty. The 

 mouth and anus are both median, situated 

 at the anterior and posterior extremities 

 respectively. 



Alimentary System. In the Apla- 

 cophora the mouth is usually a longi- 

 tudinal, rarely (ChaBtoderma) a transverse, 

 slit, situated ventrally near the anterior 

 extremity. There is a buccal cavity, with 

 in some cases an odontophore x (Fig. 565, 

 rad) in others a single chitinous tooth ; 

 sometimes teeth are entirely absent. 

 There are both salivary and buccal 

 glands. Very characteristic of the group 

 as compared with other Molluscs is 

 the presence of a straight intestine 

 devoid of coils. It has connected with 

 it either a single coecura or a double 

 row. In the Placophora the buccal 

 cavity always contains a well-developed 

 odontophore. The intestine is elongated 



1 For a description of the structure of this 

 characteristic organ, see the account of Triton, 

 infra. 



FIG. 564. Chiton, valves of 

 shell. (From the Cambridge 

 Natural History.) 



