362 MOLLUSC A. 



plants as the survivors of a multitude, which did not 

 become sufficiently gritty or poisonous. As food and bait 

 many Gasteropods are very useful; their shells have supplied 

 tools and utensils and objects of delight ; the juices of 

 Purpura and Murex furnished the Tyrian purple, more 

 charming than all aniline. 



Class III. SCAPHOPODA. 



Very different from Gasteropoda are the Scaphopoda, of which 

 Dentalium (Elephant's tooth shell) is the commonest genus. Their 

 position is uncertain. Some place them nearer bivalves, others nearer 

 Cephalopods. They burrow in the sand at considerable depths off the 

 coasts of many countries. The mantle has originally two folds, which 

 fuse ventrally, and the shell becomes cylindrical, like an elephant's tusk. 

 It is open at both ends. The larger opening (directed downwards in 

 the sand) is anterior, the concave side of the shell is dorsal. The small 

 cylindrical head bears at its extremity a mouth surrounded by numerous 

 "tentacles," while at its base there is a double cluster of ciliated con- 

 tractile processes possibly representing gills. The foot is long, with 

 three small terminal lobes. It is used in slow creeping, and is protruded 

 at the anterior opening. There are cerebral and pleural ganglia, near 

 one another in the head, pedal ganglia in the foot, and a long untwisted 

 visceral loop with olfactory ganglia near the posterior anus. Sense 

 organs are represented by otocysts beside the pedal ganglia. There is 

 an odontophore with a simple radula. The food consists of minute 

 animals. There is no heart, but colourless blood circulates in the body 

 cavity. There are two nephridial apertures, one on each side of the 

 anus ; the nephridial chamber is perforated by the intestine. The 

 sexes are separate ; the reproductive organ is simple and dorsal in 

 position ; the elements pass out by the right nephridium. The gastrula 

 is succeeded by a free swimming stage, in which there is a hint of a 

 velum and a rudimentary shell gland. 



Examples. Dentalium^ Entalium. About forty widely distributed 

 species are known. Dentalium entale occurs off British coasts. 

 The genus occurs as a fossil from Carboniferous (or perhaps 

 earlier) strata onward. 



Class IV. LAMELLIBRANCHIATA or BIVALVES. 



(Synonyms. Acephala, Conchifera, Pelecypoda, 

 Lipocephala, &c.) 



Examples. Cockles, Mussels, Clams, and Oysters. 



Lamellibranchs are bilaterally symmetrical Molluscs, in 

 which the body is compressed from side to side and the foot 



