II.] INTRODUCTION. xxix 



f 



are piled up like a cairn, partly to prevent the carcase 

 being carried away by the tide, and also because the 

 fishermen have a scruple about eating shell-fish which 

 have been fed on such carrion. On the next turn of 

 the tide the heap of stones is visited and the whelks are 

 found on the surface in great numbers, having been 

 apparently attracted by the smell of the bait, but unable 

 to get at it. 



Locomotion. — The methods by which MoUusca move 

 from place to place are exceedingly varied. Most uni- 

 valves crawl on the lower part or disk of a large fleshy 

 organ, which is the homologue of a foot and supports 

 the body. The Melampus uses this organ in an unusual 

 way, by first planting the front half, like a caterpillar, 

 and then drawing up to it the other half, and repeating 

 this alternate movement in a fashion called " looping/' 

 The celebrated French naturalist, Adanson, gave that 

 genus the name of Pedipes on account of this peculiarity. 

 Many of the aquatic univalves can swim, or rather creep 

 underneath the surface of the water, the position of their 

 bodies being reversed, with the point of the shell down- 

 wards. A few of the bivalves {e. g. species of Lepton 

 and Galeomma) sometimes walk about with their valves 

 spread out like the cover of a book when left on the 

 table by an untidy person. The Cuttles and Pteropods 

 swim as rapidly as fishes, but in a different manner — 

 viz. by taking in and expelling by means of their mantle 

 successive volumes of water, so as continually to propel 

 th^m onwards. One kind of Cuttle is said even to pos- 

 sess the faculty of flying, and to dart for a considerable 

 distance out of the water through the air like a flying- 

 fish. Most of the Acephala or bivalves have a tongue- 

 shaped organ of progression, which is muscular and 

 extremely flexible. By means of this kind of foot the 



