Invertebrate Gallery of the Indian Museum. iii 



PHYLUM VI OF THE CCELOMATA. 



MOLLUSCA. 



The Mollusca occupy the long double row of desk-cases 

 that traverses the middle of the gallery, as well as the 

 three short rows of desk-cases that run at right angles to 

 the first — one at each end of it and one bisecting it. 



A typical Mollusk — e.g., a Gastropod — is an animal 

 with a soft non-segmented body protected by a hard 

 calcareous shell into which, as into a house, the body can 

 in most cases be completely withdrawn. In the typical 

 Mollusk we can distinguish (i) a well-marked head with 

 [a) a mouth and a pair of short broad mouth-tentacles 

 or " lips ", and {b) a pair of long cylindrical cephalic 

 tentacles, upon some part of which the eyes are usually 

 carried ; and (2) a body enveloped in a fold of skin or 

 " mantle ", the under or ventral part of the body being 

 thickened to form a great solid muscular protrusible foot 

 for progression, while the thin-walled upper or dorsal 

 part contains the viscera and is permanently lodged within 

 the cavity of the shell. The shell is formed by the calci- 

 fication of the surface layer of the special fold or out- 

 growth of the wall of the dorsal part of the body already 

 spoken of as the " mantle " or pallium, which in the 

 typical mollusk completely envelopes the body and foot 

 like a cloak or skirt. The space between this mantle- 

 cloak and the body is known as the " mantle-chamber" or 

 "sub-pallial space'': it contains the breathing-organs, and 

 receives the openings of the excretory organs, and often of 

 the reproductive organs also. A good simple example of a 

 molluscan " foot ", " mantle", shell, and " mantle-chamber" 

 is furnished by the common Limpet, placed above Case 

 128. 



The internal structure of a typical mollusk may "be 

 studied in the dissections of the large Snail placed above 



