IB 



CLASSIFICATION. 



and then the young are born alive. In all cases they possess at 

 birth nearly the form they preserve through life, and do not un- 

 dergo metamorphosis. 



We have already stated that mollusks differ very much in the 

 general form of their body, and as these external modifications 

 are accompanied by corresponding differences in the degree of 

 complication of their internal organization, they enable us to di- 

 vide these animals into six natural groups. The following table 

 will give an idea of this classification : 



(CLASSES.) 



in the form of an open sack in front, from ) 

 which the head appears, surrounded by > CEPHALOPODA. 

 tentacula (Fig. 7, Page 19). ^ 



f having a 



distinct 



head, and 



the body 



not in the form of an f membranous fins in 

 open sack in front, I the form of wings 

 and the head is not | on the sides of the 

 surrounded with ten- I neck, 

 tacula. 



a fleshy foot occu-' 



T> 



PTEROPOI)A ' 



pying the ventral 



having no 



apparent > 



head 



the principal organs 

 .of motion being 



disk, or sometimes 

 ^of fins. 



having four branchiae distinct from the 

 mantle, and almost always a fleshy foot. 



having no fleshy foot f" provided with two 

 and seldom branchiae 

 distinct from the 

 mantle. 



A 



of them. J ' 



unprovided with ) 



arms and special > TUNICATA. 



organs of motion. } 



Cuvier, and many other zoologists, arrange the cirrhopoda also 

 in the branch of mollusca ; but it is now ascertained that these 

 animals belong to the branch of articulata, the third great divi- 

 sion of the animal kingdom. 



16 Into how many classes is the branch of mollus'ca divided? 



