THE CTENOPHORA. 125 



2. Order Eurystomeae. 



This order, which includes the Beroid forms, is character- 

 ized by the entire absence of tentacles and by the wide bell- 

 like stomodseum. The meridional canals send off along 

 their course numerous branching processes into the niesogloea 

 and are united around the mouth by a circular canal. To 

 this order belongs the Mediterranean genus Beroe, and the 

 genus Idyia of the northwest Atlantic. 



Relationships of the Ctenophora. The Ctenophores have been by most 

 authors assigned to the type Coelentera, on account of their jelly-like con- 

 sistency and the presence of gastro-vascular canals, and of indications of a 

 radiate symmetry. It is possible, however, that these characters are sim- 

 ply superficial, and that the group possesses but very remote affinities to 

 the Cu3lenterates. They have furthermore been regarded by some as 

 connecting links between the Coelenterates and the Turbellarians, and in 

 connection with this idea two aberrant forms may be briefly described. 

 One, Ctenoplana, is a flattened form, on the middle of whose dorsal surface 

 lies the otolith sac, and at a short distance from this are eight short rows of 

 cilia-plates, each in a slight depression. Two tentacles lie in the transverse 

 axis, and the mouth is situated at the centre of the lower surface and leads 

 into a cavity from which numerous branching gastric pouches arise with- 

 out any definite arrangement. The other form, Cceloplana, is also flat- 

 tened and creeping ; the mouth lies on the under surface and opens into a 

 wide cavity, which, as in Ctenoplana, gives origin to a number of pouches 

 which branch and give rise to a network towards the periphery of the 

 body ; a canal passes from the gastric cavity towards the dorsal surface of 

 the body, where it divides into two branches which end blindly, and lying 

 between them is a vesicle containing otoliths ; two tentacles similar to 

 those of Pleurobrachia lie in the transverse axis. In both these forms the 

 general surface of the body is ciliated, and they seem to represent inter- 

 mediate forms between the Ctenophores and Turbellaria, Ctenoplana being 

 more closely allied to the former and Cceloplana to the latter. 



There are various important structural differences, however, between 

 the Co3lentera and the Ctenophores. Among these may be mentioned 

 the structure and position of the sense-organ, the structure and position of 

 the niesogloeal muscle-fibres, the structure of the tentacles, the presence of 

 the adhesive cells which cannot possibly be homologized with nematocyst- 

 cells, and finally the early differentiation in the embryo of cells, resembling 

 the mesoderm-cells of triploblastic animals, which give rise to the muscles of 

 the tentacles and perhaps to some of the mesogloeal elements. 



It seems not improbable that the affinities of the Ctenophores would be 

 more accurately indicated in the classification if they were entirely removed 



