﻿Dec, 1857.] 



ELLIOTT SOCIETY. 



269 



acquainted with the writings of authors on the Ctenophora, are 

 entirely absent among Cydippidae. If the development of all the 

 Beroid genera be like that of Beroe punctata, as I have partially ob- 

 served it, there is also another distinction between the two groups 

 in the earlier forms of the centres of the circulatory system. For 

 in the embryo of Beroe I found this to be a single large octagonal 

 cavity, above the digestive funnel; whereas, in Bolina, it consisted 

 of three inter-communicating sinuses, a central one marked ra, in 

 fig. 5, pi. 14, and two lateral sinuses, marked a, in fig. 4, of the 

 same plate. In the adult Beroe, also, there is an apparatus of 

 many tortuous horizontal tubes inter-communicating between the 

 vertical ambulacral tubes. Now no such apparatus has been 

 traced, so far as I am aware, in any Cydippidae, and indeed I am 

 led to think, by my own observations, that they appear very late 

 in the growth, even of Beroe. It would be interesting to know 

 whether this difference is also constant for the two groups. 



What I may call the morphological tendencies also of the 

 groups is different. Among Beroidae there is a tendency to an 

 elongation of the vertical axis, while the reverse is the case among 

 Cydippidae, where lateral dilatation is the manifest tendency, as 

 in Bolina and Cestum.* It is the superior pole of the body also 

 in Beroe which exhibits complexity in its structure, while among 

 Cydippidae this pole is comparatively simple and the sides of the 

 body, on the contrary, have a marked disposition to assume com- 

 plex forms. A further knowledge of these differences will pro- 

 bably result in the establishment of two sub-orders in the group of 

 Ctenophora, viz : the Cydippidae and the Beroidae proper. 



If the view of homologies herein suggested be correct, the 

 Beroidae would constitute the higher group. For in them the 

 tentacula have disappeared, the area of the sense-capsule become 

 more complicated and the superficial system of tubes has taken 

 on a much larger development. The tentaculum appears in fact 

 to be a badge of inferiority. It exhibits a great numerical de- 

 velopment among the members of the lower orders, and it is most 

 developed in Cydippe and Pleurobrachia, the lowest genera among 

 Ctenophora. It gradually dwindles away among the higher genera 

 of the group of Cydippidae, and at last entirely disappears among 

 Beroidae. The more complicated condition of the sense-capsule 



* This difference may be, to some extent, analogised with that which obtains 

 between the tendency to dilatation among Echini and the tendency to elongation 

 among Holothuridae. 



