696 



ZOOLOGY 



SECT. 



sometimes partly, sometimes completely, covered over .by the 

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



Fia. 609. Chiton spinosus, dorsal view. 

 (From the Cambridge Natural History.) 



FIG. 610. Chiton, ventral view. an. anus ; 

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

 mo. 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 (Chcetoderma) a transverse, slit, situated 

 ventrally near the anterior extremity. There 

 is a buccal cavity, with a radula 1 in some 

 cases (Fig. 612, rad), and in others a single 

 tooth supporting smaller den- 



F,<,6ii.-c*iton, valves of 

 shell. (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). 



