MOLLUSCA. 305 



Division of the Mollusca into Six Classes{l). 



The general form of the body of the Molhisca, being in pro- 

 portion to the complication of their internal organization, in- 

 dicates their natural division(2). 



The body of some resembles a sac open in front, containing the 

 branchiae, whence issues a well developed head crowned with 

 long and strong fleshy productions, by means of which they 

 crawl, and seize various objects. These we term the Cepha- 

 lopoda. 



That of others is closed ; the appendages of the head are 

 either wanting or are extremely reduced j the principal or- 

 gans of locomotion are two wings or membranous fins, situated 

 on the sides of the neck, and which frequently support the 

 branchial tissue. They constitute the Pteropoda. 



Others again crawl by means of a fleshy disk on their belly, 

 sometimes, though rarely, compressed into a fin, and have 

 almost always a distinct head before. We call these the Gas- 

 teropoda. 



A fourth class is composed of those where the mouth re- 

 mains hidden in the bottom of the mantle, which also encloses 

 the branchice and viscera, and is open either throughout its 

 length, at both ends, or at one extremity only. Such are our 



ACEPHALA. 



A fifth comprises those, which, also inclosed in a mantle and 

 without an apparent head, have fl.eshy or membranous arms, 

 furnished with cilia of the same nature. We term these 

 Brachiopoda. 



Finally, there are some, which, although similar to the other 

 Mollusca in the mantle, branchiae, &c., differ from them in 

 numerous horny and articulated limbs, and in a nervous sys- 

 tem more nearly allied to that of the Articulata. They will 

 constitute our last class, or that of the Ciuuhopoda. 



(1) M. cle Blainville has substituted the name of Malacozouires for that of Mol- 

 lusca, separating from them the Chitons and Cirrhipoda, which he calls Maknto- 

 zoaires. 



(2) The whole of this arrangement of the Mollusca, and most of the secondary 

 subdivisions, belong exclusively to me. 



Vol. II.— 2 O 



