714 



ZOOLOGY 



SECT. 



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

 opens (Fig. 602). 



In Chiton (Figs. 599 and 600) the body is dorso- ventral ly com- 

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

 (narrow in Chitonellus) 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 superficial and a deeper, the latter 

 formed of compact calcareous substance, the former perforated by 



Fio. 598.— Neomenia 

 carinata. a. anus ; 

 gr. ventral groove ; 

 m. mouth. (Prom the 

 Cambridge Natural 

 History.) 



Fig. 599.— Chiton spinosus, dorsal view. 

 (From the Cambridge Natural History.) 



pip 



jnutil 



alert. 



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

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

 mo. mouth ; pip. 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 (Figs. 600 and 606, cten.) to the number of 



