ZOOLOGY 



SECT. 



.771 



FIG. 5iiO. Chsetoderma 

 nitidulum. <t. anus ; 

 m. mouth. (From the 

 Camljridije Natural His- 

 tori/.) 



short and thick. In most instances there is little difference in 

 external appearance between the anterior and posterior ends. In 

 Chcetoderma (Fig. 560) alone is there a distinct head, separated off 



from the body by a constriction, as well as 

 a posterior cloacal region which is similarly 

 marked off. A shell is completely absent. 

 The mantle covering the surface possesses 

 a cuticle, in the substance or on the surface 

 of which are spicules of calcified material. 

 Along the middle of the ventral surface 

 runs, in most instances, a groove, in some 

 cases merely represented by a narrow strip 

 from which the cuticle and spicules are 

 absent. The ventral groove, when present, 

 usually contains a slight longitudinal ridge, 

 and this is all that in these primitive forms 

 represents the foot, an organ so highly de- 

 veloped in other Molluscs. In Choetoderma 

 it is entirely absent. With the ventral 

 groove is connected in front an anterior 

 ciliated groove, while behind it is in direct 

 communication with the cavity of the cloaca. 

 In Proncomenia ctenidia are absent. In the remaining genera 

 there is either a pair or a circlet of gills situated in the cloaca 

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

 opens (Fig. 565). 



In Chiton (Figs. 562 and 563) the body is dorso-ventrally com- 

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

 (narrow in Chitonellus) which acts not only as an organ for effect- 

 ing 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 must 

 remarkable external feature of Chiton is the 

 presence on the dorsal surface of a calcareous 

 shell (Figs. 562 and 564) made up of no fewer 

 than eight transversely elongated pieces or 

 valves, arranged in a longitudinal row, arti- 

 culating together and partly overlapping one 

 another. These valves are primitive, being 

 direct developments of a series of shell-glands 

 of the embryo and not secondary structures 

 like the shells of other Molluscs. They are 

 sometimes partly, sometimes 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 numerous vertical canals for the 

 lodgment of the sense-organs to be presently referred to. External 



Fio. 561. Neomenia 

 carinata. a. anus ; 

 fir. ventral groove ; 

 m. mouth. (From the 

 Co/nib rid ye Kutt 1 i'"l 

 History.) 



