30 



MOLLUSCA. 



The animals live buried in mud, and creep about slowly by means 

 of the foot. The young swim about for some time as larvae, 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). 



Order. Solenoconchae. 



Fam. Dentalidae. Dentalium entails L., D. elephantinum L., Mediterranean 

 and Indian Ocean. 



Class II. GASTROPODA.* 



Mollusca with distinct head, often bearing tentacles ; a ventral 

 muscular foot and undivided mantle, which frequently secretes a simple 

 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 



Eyes at the extremity of the body ; its f Orm 



and size presents 



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 intx 



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

 Kiister. Niirnberg, 1837-1865. 



Sowerby. " Thesaurus conchyliorum, or figures and descriptions of sheila." 

 London, 18321862. 



Keeve. " Conchologia iconica, etc." London, 1842-1862. 



H. and A. Adams. " The genera of the recent Mollusca," 3 voJs. London, 1858. 



H. Troschel. " Das Gebiss der Schnecken." 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. 



FIG. 60S. Helix 



pom a Ha. 



long tentacles ; Pe, foot. 



numerous modifications. 





