A Cephalophorous Mo Husk. 59 



A CEPHALOPHOROUS MOLLUSK 



(Sycotypus canaliculatus) . 



External Anatomy. Examine an animal enclosed 

 within its shell and note the following points : 



The shell, unlike that of the Lamellibranch, consists of but 

 a single piece {Univalve). It is twisted towards the right, 

 around a central axis, the columella, and makes several 

 revolutions or whorls from its apex or spire, to its lower 

 opening or " mouth." The side of the mouth nearer the 

 columella is called the internal lip, while the outer, thin, 

 convex edge is the outer lip. Extending from the mouth 

 downwards (away from the spire) is an elongated groove, 

 the canal. The line of union between two whorls is called 

 the suture. Lines of growth extend as parallel series, at 

 right angles to the suture, and parallel to the edge of the 

 outer lip. Frequently the shell is covered externally by a 

 felt-like horny epidermis. 



Draw the shell from the side, naming the parts. 



From the " mouth ' of the shell the soft parts of the 

 snail may be protruded, the large foot being the last to 

 disappear during contraction and the first to leave the shell 

 as the animal again expands. To the upper side of the 

 foot a horny lid, or operculum, is attached, which may effec- 

 tually close the opening of the mouth. 



As the animal crawls about, the shell is carried upon its 



