XI 



PHYLUM MOLLUSCA 



281 



and the razor-fish (So/en) jerks itself forward by suddenly 

 withdrawing its very large foot, and thus ejecting water 

 through the siphons. The only parasitic genus is Ento- 

 valva, found in the gullet of a Holothurian. 



Pelecypoda are abundant both in fresh water and the sea ; 

 the marine forms are mainly littoral. None are pelagic or 

 terrestrial. 



2. THE AMPHINEITRA 



A class of molluscs which comprises only a small number of repre- 

 sentatives, most of them of rare occurrence and of simple organisation, 

 is the Amphineura. With the exception of the Chitons these have no 

 shell and are devoid of a foot, so that though probably related to the 

 more typical molluscs, and to be referred to the same phylum, they are 

 wanted in some of the most characteristic features exhibited by the 

 members of the other classes. All the 

 Amphineura are bilaterally symmetri- 

 cal, more or less elongated molluscs, 

 with the mouth at the anterior and the 

 anus at the posterior end. 



The commonest, as well as the most 

 highly organised, of the Amphineura 

 are the Chitons, marine molluscs which 

 are to be found adhering firmly, like 

 limpets, to rocks and stones on the 

 seashore, or in deep water. 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 creep- 

 ing movements, but also as a sucker for 

 enabling the animal when at rest to FIG. 168 Chiton spinosus, dorsal 

 adhere firmly, like a limpet, to the sur- Datura? H^to'ry.) * 

 face of a rock. The most remarkable 



external feature of the Chiton is the presence on the dorsal surface of 

 a calcareous shell (Fig. 168), made up of no fewer than eight trans- 

 versely elongated pieces or valves, arranged in a longitudinal row, artic- 



