CLASSIFICATION 43 
in which the radula is wanting and the arms are 
united by a membrane, while fins are developed on 
the body. Some pelagic (Cirroteuthis) and deep-sea 
forms (Opisthoteuthis) belong here, and possibly also 
the fossil Palgoctopus. (b) TRAcHYGLossA, the 
members of which have a radula, but no true fins. 
The Common Octopus (Polypus vulgaris) and the 
Argonaut are familiar examples of this tribe. 
Of the interrelationships of these five classes it 
is impossible to say much in the present state of our 
knowledge, beyond that the first four are the more 
closely related. 
