CHAPTER XVI 



CLASSES SCAPHOPODA AND PELECYPODA 



CLASS SCAPHOPODA 



Head rudimentary, mantle edges ventrally concrescent, 

 forming a tube opening before and behind, 

 and covered with a shell of the same shape ; 

 sexes separate. 



The Scaphopoda form a small but very 

 distinct class, whose organisation is decidedly 

 of a low type. The body is usually slightly 

 curved, the concave side being the dorsal ; 

 muscles near the posterior end attach the 

 body to the shell. The foot, which can be 

 protruded from the anterior or wider aper- 

 ture, is rather long, pointed, and has some- 

 times two lateral lobes (^Bentalium^^ some- 

 times a terminal retractile disc (^Siphonoden- 

 taliuni)^ sometimes a retractile disc with a 

 central tentacle (^Pidsellur)i). The cephalic 

 region, as in Pelecypoda, is covered by the 

 mantle. The mouth is situated on a kind of 

 proiection of the pharynx : the buccal mass. 



Fig. 299. -Anatomy of ^ -^ . . . i / ^ c^^n^ ■ i 



Dentalium: a, auteriov Containing the radula (p. 2db), IS at the 

 aperture of mantle ; /, j^^^gg Qf ^\^q f^Qj-^ ^j^(-[ ^]-^g intestine branches 



k, kidney; ^, ^liver! forward from the front part of the stomach. 



thierT) ^^''^^^'^''' The liver (Fig. 299) is paired, and consists 



of a number of symmetrical, radiating coeca. 



There are no eyes, but on each side of the mouth are small 



bunches of exsertile filaments (captaculd)^ which appear to act as 



444 



