XII 



PHYLUM MOLLUSCA 



695 



607. Chaetoderxna 

 nitidulum. a. anus ; 

 m. mouth. (From the 

 Cambridge Natural His- 

 tory.) 



2. GENERAL ORGANISATION. 



External Features. The Aplacophora are distinguished by 

 their worm-like body, sometimes elongated and narrow and capable 

 of being coiled into a spiral, sometimes comparatively short and 

 thick. In most instances there is little differ- 

 ence in external appearance between the an- 

 terior and posterior ends. In some species of 

 Chcetoderma (Fig. 607) alone is there in sexually 

 mature specimens a "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. In 

 many cases the dorsal surface is beset with 

 uni- or multi-cellular epidermal papillae. 

 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 groove with its 

 contained ridge is all that in these simple forms represents the foot, 

 an organ so highly developed in other Molluscs. In Chsetoderma it is 

 entirely absent. With the ventral groove is connected in front 

 an anterior ciliated groove, while behind it is in direct communica- 

 tion with the cavity of the cloaca. 



In Proneomenia ctenidia are absent. In the remaining genera 

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

 form of simple or complex folds situated in the 

 cloaca a cavity at the posterior end of the body 

 into which the anus opens (Fig. 612). 



In Chiton (Figs! 609 and 610) the body is 

 dorso-ventrally compressed, 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 



Cambridge Natural P P,. * , & . , , J ,, 



History.) 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 provided 

 with eyes or tentacles.. The most remarkable external feature of 

 Chiton is the presence on the dorsal surface of a calcareous shell 

 (Figs. 609 and 611) made up of no fewer than eight transversely 

 elongated pieces or valves, arranged in a longitudinal row, articu- 

 lating together and partly overlapping one another. They are 



FIG. 608. Neomenia 

 carinata. a. anus ; 

 gr. ventral groove ; 

 m. mouth. (From the 



