xii PHYLUM MOLLUSCA 713 



ORDER 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 

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

 of ctenidia. 



This order includes Neomenia, Proncomenia, 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 F 

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



rm i 11 Cambridge Natural His- 



The ventral groove, when present, usually tory .) 

 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 Cha^toderma 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 '/. 



