CHAP. XVI SCAPHOPODA 445 



tactile organs for the seizing of food. There is no special respir- 

 ing apparatus, heart or arterial system, breathing being conducted 

 by the walls of the mantle. The nervous system has already 

 been described (p. 205). 



Two kidneys open on either side of the anus. The genital 

 gland is large, occupying nearly all the posterior part of the body, 

 the sexual products being emitted through the right kidney. 

 The veliger has already been figured (p. 131, Fig. 44). The 

 embryonic shell is formed of two calcareous laminae, which sub- 

 sequently unite to form the tube. 



With regard to their general relationships, the Scaphopoda 

 resemble the Gasteropoda in their univalve shell, and in the 

 possession of a radula ; while the pointed foot, the non-lobed 

 velum in the veliger, the generative system, the bilateral symmetry 

 of the organs generally, and the absence of any definite head, 

 eyes, or tentacles, are points which approximate them to the 

 Pelecypoda. 



The Scaphopoda are known from Devonian strata to the 

 present time. They are found at a depth of a few fathoms to 

 very deep water. The only three genera are Dentalium, Siphono- 

 dentaliuin (subg. Cadulus), and Pulsellum, which difler in the 

 structure of the foot, as described above. 



CLASS PELECYPODA 



Cephalic region rudimentary, mantle consisting of two sym- 

 metrical right and left lobes, covering the body and secreting a 

 bivalve shell hinged at the dorsal margin; no radula, sexes usually 

 separate. Eeference has already been made to the reproductive 

 system (p. 145), breathing organs (p. 164 f.), mantle (p. 172), 

 nervous system (p. 205), digestive system (p. 237 f.), and nomen- 

 clatm-e of the various parts of the shell (p. 269 f.). 



The shape of the shell, in many Pelecypoda, involving as it 

 does the position, size, and number of the adductor muscles, is 

 probably due to mechanical causes, depending on the habits and 

 manner of life of the individual genus. Thus in a typical 

 dimyarian or two-muscled bivalve, e.g. Mya (Fig. 300, A), the 

 adductor muscles lie well towards each end of the long axis of the 

 shell, with the hinge about midway between them. In this 

 position they are best placed for effectually closing the valves. 



