68o 



CHAPTER XXXV. 

 SUB-KINGDOM MOLLUSCA. 



GENERAL CHARACTERS OF THE MOLLUSCA GENERAL CHARACTERS 



OF THE LAMELLIBRANCHIATA. 



The Mollusca may be defined as soft-bodied, bilaterally symmetrical, 

 not definitely segmented animals. The anterior part of the body is 

 very generally developed into a distinct head, bearing one or more pairs 

 of soft tactile processes or tentacles. The mouth is anterior, the ali- 

 mentary canal is completely shut off from the general cavity of the 

 body, and the anus is primitively posterior. The nervous system con- 

 sists of a small number of paired ganglia. A distinct vascular sys- 

 tem and a systemic heart are present. One or more pai?'s of kidiieys 

 (sometimes a single kidney) are present as saccular organs (" ne- 

 phridia "), which open internally into the body-cavity, and communi- 

 cate with the exterior by a pore placed near the anus. Commonly 

 the7'e is an external or internal " shell.'''' 



The body of a Mollusc exhibits a distinct dorsal and ventral 

 surface, and a right and left side. The dorsal surface is covered 

 by a fold of the integument which constitutes what is called the 

 "mantle" or "pallium," and which may be greatly expanded later- 

 ally, or may form a complete sac enclosing all the viscera. The 

 so-called "mantle-cavity" or "pallial chamber" is the space in- 

 cluded between the lateral prolongations of the mantle and the 

 sides of the body. 



From the ventral side of the body there is, typically, developed 

 an unpaired median muscular mass, which constitutes what is called 

 the "foot." The foot (fig. 551, f) may show a distinct division 

 into an anterior, a middle, and a posterior region ; and it is often 

 furnished with distinct lateral prolongations (" epipodia "). The 

 foot undergoes remarkable modifications in different groups of the 

 Mollusca. 



The alimentary canal commences at the mouth (fig. 551, m), 



