30 



MOLLUSCA. 



The animals live buried in niud, and creep abcmt slowly by means 

 of the foot. The young s\vim about for some time as larva?, provided 

 with ciliated tuft and ciliated collar ; then acquire a shell, which is 

 almost bivalve, a velum, and foot; the shell subsequently becomes 

 tubular (fig. 507). 



Fain. Dentalidae. 

 and Indian Ocean. 



Order. Solenoconchae. 



Dcntdllvm entalix L., D. elephant/ mini L.. Mediterranean 



Class II. GASTROPODA. 



Mollusca with distinct head, often bearing tentacles ; a ventral 

 muscular foot and undivided mantle, which frequently secretes a ximpli' 

 plate-shaped or spirally twisted shell. 



The anterior part of the body or head usually bears two or four 



tentacles and two 

 eyes, which are 

 placed sometimes at 

 the apex, usually at 

 the base of a pair of 

 tentacles (fig. 508). 

 The muscular foot 

 projects from the 

 ventral side of the 

 its form 

 presents 



Pe 



FIG. 508. Helix poniutiu. 0, Eyes at the extremity of the body : 

 long tentacles ; 1'c, foot. J ' 



and size 



numerous modifications. As a rule it has a broad and long plantar 

 surface ; but in the Heteropoda it has the form of a vertically extended 

 fin. The shape of the body depends on the position and form of the 

 mantle. The latter is placed like a cap on the dorsal surface, 

 and consists of a more or less considerable fold of the dorsal integu- 

 ment ; its edge is usually thickened, sometimes also prolonged into 



* Martini and Chemnitz. Conchylien-Cabinet. 12 Bde. Herausgegeben von 

 Kiister. Niirnberg, 1837-1865. 



Sowerby. " Thesaurus conchyliorum, or figures and descriptions of shells/' 

 London, 183218(52. 



Reeve. " Conchologia iconica. etc." London, 1842-1S62. 



H. and A. Adams. " The genera of the recent Mollusca," 3 vols. London. 1 858. 



H. Troschel. " Das Gebiss cler Schuecken." Berlin, 1856-1878. 



Woodward. " Manual of the Mollusca." 2nd ed., London, 1868. 



Fol. " Etudes sur le developpement des Mollusques." I. and II. 



C. Eabl, " Ueber die Entwickelung der Tellerschnecke/' Morphol. Jahrbuch. 

 Tom. V. 1819. 



