HOMOLOGIES. 199 



somatic cavity in Beroe suddenly diminishes in width towards the aboral end, and is there 

 provided with a pair of valve-Hke folds (s), so that the entire tract admits of being distinguished 

 into two regions, it is nevertheless as continuous as in Hydra. 



The advocates of the actinozoal nature of the Ctenophora see in the canal-system of a Beroe 

 or a Cydippe the radiating chambers of an Actinia separated from one another by partitions of 

 relatively enormous thickness. I do not desire to dispute the correctness of this view ; we 

 have already compared a hydroid with an actinozoon, and have seen in the radiating canals of 

 a hydroid medusa the homologues of the radiating chambers of an Actinia ; so that even though 

 the Ctenophora be truly HyDROzoA, we must expect to find in them the same points of agree- 

 ment with the AcTiNOZOA which we have endeavoured to demonstrate for the other hydrozoal 

 orders. 



Now, the fact of the radiating canals being widely separated from the axial cavity instead of 

 being adnate to it, is exactly the point which essentially distinguishes a hydrozoon from an 

 actinozoon, and the fact of the intervening space being in the Ctenophore obliterated by the 

 interposition of a voluminous gelatiniform mass does not alter this relation, for it is exactly wdiat 

 we find in the atrial region of an ordinary hydroid medusa, while it is distinctly expressed in the 

 gonopbore of Clavatella (see below, page 216), where the free or manubrial portion of the 

 umbrella is rudimental, and the whole gonophore, apart from the marginal tentacles, becomes 

 comparable to the atrial region of an ordinary hydroid medusa. 



The two accessory canals of Beroe run, it is true, close upon the walls of the axial cavity until 

 they leave these to throw themselves into the circular canal ; but this fact cannot, in opposition to 

 the greatly preponderating hydrozoal features, be used as an argument for the actinozoal nature 

 of the Ctenophora. 



These accessoiy canals are not represented in the hydroid, while the Beroe further differs from 

 the hydroid in the presence of the two short aboral canals by which the aboral end of its somatic 

 cavity communicates with the outer world {t), as well as in the disposition of its so-called nervous 

 system and sense organs, and in its characteristic bands of vibratile lamellge (y), all which features 

 are among the special characteristics of the order, and in no way justify the absorption of the 

 Ctenophora into the Actinozoa. 



In this attempt to determine the true affinities of the Ctenophora, I have taken Beroe 

 instead of Cydippe or other ctenophorous genus as the subject of comparison, not only because 

 Beroe is a typical ctenophorous form and of comparatively simple structure, but because I have 

 myself made its anatomy and development a subject of special study.' 



' " Ou the Structure and Development of Beroe," ' Proc. Koy. Soc. Ediub.,' 1862. 



