18 



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 i 

 which the head appears, surrounded by > Cephalopoda, 



f having a 



distinct 

 head, and 

 the body 



tentacula [Fig. 7, Page 19). 





not in the form of an f membranous fins in ^ 



open sack in front, the form of wings I p 



and the head is not | on the sides of the r^TEROPODA« 



surrounded with ten- neck, J 



taeula. | 



•{ a fleshy foot oceu- 

 the principal organs pying the ventral 

 ^of motion being face of the body, 



and in form of a 

 disk, or sometimes 

 ^of fins, 

 f having four branchiae distinct from the 



t mantle, and almost always a fleshy foot. ^ 



apparent ^having no fleshy foot C provided with two"^ 



> Gasteropoda. 



ACEPHALA. 



head 



nd seldom branchiae ciliated arms which l •» 

 istinct from the J they have in place ,• 



an 



distinct 



mantle. 



of them. 



unprovided with 

 arms and special 

 organs of motion 





TUNICATA. 



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? 



