216 ACTINOZOA. 



Family 2. Ctathaxonid.e. 



Corallum simple. Se'pta complete. No 

 dissepiments or tabulse. 

 Family 3. Cyathophtllid^. 



Corallum simple or composite. Septa in- 

 complete. Tabulte, in general, present. 

 Family 4. Cystiphtllid^. 



Corallum simple ; composed chiefly of a 

 vesicular mass, with but slight traces of 

 septa. 



5. Order 4 : Ctenophora. — The leading 

 characters of the Ctenophora, or oceanic Acti- 

 nozoa, have been already, to some extent, sketched 

 out in the account given of Pleurobrachia, se- 

 lected as the type of the group, and in the com- 

 parative survey which has been taken of the prin- 

 cipal organic systems of the present class. 



The more striking modifications which appear 

 within the limits of the order have reference 

 chiefly to size, the general form of the body, the 

 several parts of the canal system, and the struc- 

 ture and arrangement of the tentacles. 



The ordinary dimensions of Pleurobrachia are 

 by many other Ctenophora frequently exceeded, a 

 common British species of Bolina often attaining 

 a long diameter of two or three inches, while 

 Beroe sometimes reaches the size of a large lemon. 



In Pleurobrachia alone does the form of the 

 body approach the spheroidal {fig. 39, e). Its 

 axis is somewhat lengthened in the Beroidce, so 

 that the animal, when seen in profile, appears 

 more or less ovate (c). In Cestuni, on the other 

 hand, elongation takes place to an extraordinary 

 extent, at right angles to the direction of the 



