xii PHYLUM MOLLUSCA 713 



ORDEK 1. PLACOPHORA. 



Amphineura with a broad foot, and with a shell which consists 

 of eight transverse valves. There is a row of ctenidia on either side. 

 This order includes the Chitons. 



ORDER 2. APLACOPHORA [SOLENOGASTRES]. 



Amphineura with an elongated body covered completely by the 

 mantle, without shell, but with calcareous spicules. There is no 

 foot, but generally a ventral longitudinal groove along which 

 usually runs a low ciliated ridge. In some there is a posterior 

 cavity (cloaca or mantle-cavity), containing a pair or a circlet 

 of ctenidia. 



This order includes Neomenia, Proneomenia, Chcetoderma, and 

 a number of other genera. 



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, some- 

 times comparatively short and thick. In 

 most instances there is little difference in 

 external appearance between the anterior 

 and posterior ends. In Chcetoderma (Fig. 597) 

 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 calcined material. 

 Along the middle of the ventral surface runs, 

 in most instances, a groove, in some cases 

 merely represented by a narrow strip from FIG. 597.-ciiaetodenna 



. . . J . , J , . r nitiduluxn. a. anus ; 



which the cuticle and spicules are absent. m. mouth. (From the 

 The ventral groove, when present, usually %**** 

 contains a slight longitudinal ridge, and this 

 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 situated in the cloaca 



VOL. I z z 



