CHAPTER XXL 



MOLLFSCA PEOPEB. 



Thb higher Mollusca or MoUusca proper comprise those 

 members of the sub-ldngdom in which the nervous system 

 consists of three principal pairs of ganglia ; and there is 

 always a well-developed heart, consisting of at least two 

 chambers. 



In this division are included the following classes : 



1. JOamellihranchiata, without a distinct head. 



2. Gasteropoda, ) -.i, ■,■,-,,• ■, , ^. , 



3. Pteropoda, [ ^^*^ ^ "^^"^T* ^^^'? ^""^ * masticatory 



4. Cephalopoda, ) apparatus or odontophore." 



Class I. LA3iBLUBEATfCHL4.TA. — ^These are well known as 

 bivalve shell-fish, such as mussels, oysters, scallops, etc., and 

 they are all either marine or inhabitants of fresh water. They 

 are distinguished from the other Mollusks by having no dis- 

 tinct head, and by having the body more or less completely 

 protected by a bivalve-shell composed of two pieces. They 

 are called Lamellihranchiata (Lat. lamella, a plate ; Gr. hrag- 

 chia, gill), from the fact that the organs of respiration are in 

 the form of leaf-like gills or branchise, two of which are placed 

 at each side of the body, constituting what ia':known in the 

 oyster as the " beard." The body of the Lamettibranchiata 

 is more or less completely enclosed in an expansion of the 

 integument which constitutes the " mantle," and which is 

 divided into two halves or " lobes," which are placed on the 

 sides of the animal, and secrete the shell. The shell, there- 

 fore, of the true bivalves is composed of two valves, which 

 are " right " and " left," and not " dorsal " and " ventral," as 

 in the Jirachiopoda. Moreover, the valves of the shell are 

 usually of the same size, so that the shell is " equivalve," and. 



