IV PHYLUM OELENTERATA 125 



means of its two reversed pairs of hind legs. The sea- 

 anemone appears to have fixed itself when young to the 



Fig. 64. — Cancrisocia living as a commensal on the back of a crab. (After Verrill.) 



shell, and afterwards, by its growth, spread over the back of 

 the crab, taking the place of the shell. 



4. THE CTENOPHORA 



The Ctenophora or comb-jellies are a group of free- 

 swimming, gelatinous, transparent animals which occur, some- 

 times in enormous numbers, in the surface waters of the sea. 

 The animal (Fig. 65) has the appearance of a mass of clear 

 jelly, usually of a globular shape ; and no pulsating move- 

 ments, such as those by means of which a Medusa propels 

 itself, are to be observed. Running over the surface, nearly 

 from pole to pole of the globular body, there will be observed 

 a series of eight bands of flashing points of light. These are 

 found, when examined more closely, to consist of rows of 

 long cilia, which run at right angles to the long axis of the 



