ZOOLOGY 



SECT. 



sometimes partly, sometimes completely, covered over by the 

 mantle. Each valve consists of two very distinct layers, a more 



Tin I lit 



a 



a 



et en- 



FIG. 609. Chiton spinosus, dorsal view. FIG. 610. Chiton, ventral view. an. anus ; 

 (From the Cambridge Natural History.) cten. etenidia ; ft. foot ; mant. mantle edge ; 



tno. mouth ; pip. palp. (After Pelseneer.) 



superficial and a deeper, the latter formed of compact calcareous 

 substance, the former perforated by numerous vertical canals for 



the lodgment of the sense-organs to be,pre- 

 sently referred to. External to the valves 

 the dorsal integument (mantle) of Chiton 

 and its allies is usually beset with a number of 

 horny or calcified tubercles and spicules. 

 The mantle develops only very slight lateral 

 flaps, and under cover of these, in mere 

 grooves which represent the mantle-cavities 

 of other Molluscs, are a series of small 

 ctenidia (Figs. 610 and 616, cten.) to the 

 number of from fourteen to eighty. The 

 ]-, mouth and anus are both median, situated 

 at the anterior and posterior extremities 

 respectively. 



Alimentary System. In the Aplaco- 

 phora the mouth is usually a longitudinal, 

 rarely (Chatoderma) a transverse, slit, situated 

 ventrally near the anterior extremity. There 

 is a buccal cavity, with a radula 1 in some 

 cases (Fig. 012, rad), and in others a single 

 FI. on. -chiton, valves ,,f chrtinous tooth supporting smaller ^ den- 

 sheii. (From the Cambridge tides : sometimes teeth are entirely absent. 



There are both salivary and buccal glands. 



1 For a description of the structure of this characteristic organ see the 

 account of Triton (p. 708). 



