ri4 



ZOOLOGY 



SECT. 



— a cavity at the posterior end of the body into which the anus 

 opens (Fig. 602). 



In Ghiton (Figs. 599 and 600) the body is dorso-ventrally com- 

 pressed, convex above, and presents below a broad flat foot 

 (narrow in Chitoncllus) which acts not only 

 as an organ for effecting creeping movements, 

 but also as a sucker for enabling the animal 

 when at rest to adhere firmly, like a Limpet, 

 to the surface of a rock. The head region is 

 not distinctly separated off, and is not pro- 

 vided with eyes or tentacles. The most re- 

 markable external feature of Chiton is the 

 presence on the dorsal surface of a calcareous 

 shell (Figs. 599 and 601) made up of no fewer 

 than eight transversely elongated pieces or 

 valves, arranged in a longitudinal row, articulating together and 

 partly overlapping one another. They are sometimes partly, some- 

 times completely, covered over by the mantle. Each valve consists 

 of two very distinct layers, a more supei-ficial and a deeper, the latter 

 formed of compact calcareous substance, the former perforated by 



Fic. .'lOs.— Weomenia 

 carinata, a. anus ; 

 f/r. ventral groove ; 

 III. mouth. (Fi-oni the 

 CionOridge Natural 

 History.) 



Vvi. 611!). — Cliiton spinoBus, dorsal view. 

 (From the Cambridge Natural History.) 



pip 



maul 



cterL 



Piii. 600.— Chiton, ventral view. an. anus ; 

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

 mo. mouth ; ifl}^ palp. (After Pelseneer.) 



numerous vertical canals for the lodgment of the sense-organs to 

 be presently referred to ; the former alone represents the shell of 

 other Molluscs. 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 are a series 

 of small ctenidia (Fig.s. 600 and 606, cte7i.) to the number of 



