ELEPHANT 'S- TOOTH SHELLS, ETC. 



629 



is probably taken by a double row of retractile filaments, each terminating 

 in a knob-like thickening, that encircle the base of the snout. The heart is 

 quite rudimentary. The foot lies under the snout, and is a long, cylindrical 

 body directed forwards and terminating in three lobes. The nervous system 



1 S CG 



Fig* 11. DENTALIUM SHELL AND DIAGRAM OF ANATOMY. 



AC. Alimentary canal. F t Foot. M', Mantle. S'. Shell. 



CG, Cephalic ganglia. L, Liver. PQ, Pedal ganglia. TT, Tentacles. 



EA, Efferent aperture,, M, Mouth. S, Snout VG, Visceral ganglia. 



is simple and resembles in general arrangement that of the Pelecypoda 

 already described (p. 620), except that the cerebral and pleural ganglia are 

 not fused. The Scaphopoda live in mud, and feed on the small organisms 

 they find in it. 



CLASS IV. GASTROPODA. 



In this class are included mollusca like the snail, whelk, etc., that have a 

 distinct head, and, along the underside of the body, a muscular foot on which 

 they creep ; whilst the visceral mass situated above is sometimes naked, but 

 more often covered with a shell, composed of a single piece. 



The organs of the body are asymmetrical, i.e., those on one side of the 

 body are not matched by corresponding ones on the other. 



The shell is essentially a longer or shorter hollow cone. In some, such as 

 the limpet, it is a simple cone, but in by far the greater number it is an 

 elongated cone, coiled round and round spirally, each coil forming a whorl, 

 the last being the body-whorl. Nearly all spiral shells are dcxtral; that is to 

 say, when placed with the mouth uppermost, and the apex directed away 

 from the observer, the mouth lies to the right hand of the axis of the shell. 

 Some are wound in the opposite direction, and are sinistral. Reversed 

 varieties of shells normally dextral, or sinistral, are met with. 



The axis or columella of the shell is sometimes hollow or umbllicated 

 (the hollow itself being called the umbilicus), sometimes the whorls are 

 closely coiled, and solid pillar of shell results. The apex or extreme top of 

 the shell generally differs in markings and other features from the rest, and 

 offers important characters. This nudeus or protoconch is the portion 

 formed in the egg, hence it is also known as the embryonic shell. 



