28o MANUAL OF ZOOLOGY SECT. 



slow movements of the foot, and some are permanently 

 fixed during adult life by the byssus. The Scallops, 

 however, swim freely by clapping the valves together. The 

 Cockles (Cardium\ Trigonia, etc., jump by sudden move- 

 ments of the foot, and the Razor-fish (So/en) jerks itself 

 forward by suddenly withdrawing its foot and thus ejecting 

 water through the siphons. The only parasitic genus is 

 Entovalva, 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 AMPHINEURA 



The Amphineura are a class of Molluscs which comprises only a 

 small number of representatives, most of them of rare occurrence and of 

 simple organisation. With the exception of the Chitons they have no 

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

 more typical Molluscs, and referable to the same phylum, they are 

 wanting in some of the most characteristic features exhibited by the 

 members of the other classes. All the Amphineura are bilaterally 

 symmetrical, 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 Amphi- 

 neura, are the Chitons marine Molluscs which are to be found adhering 

 firmly, like Limpets, to rocks and stones on the seashore. The body is 

 dorso*ventrally compressed, convex above, and presents below a broad 

 flat foot (narrow in Chifonellus], which acts not only as an organ for 

 effecting creeping movements, but also as a sucker for enabling the 

 animal when at rest to adhere firmly to the surface of a rock. The 

 most remarkable external feature of the Chiton is the presence on 

 the dorsal surface of a calcareous shell (Fig. 156), made up of no 

 fewer than eight transversely elongated pieces or valves, arranged in 

 a longitudinal row, articulating together and partly overlapping one 

 another : these are somtimes 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 sub- 



