MANUAL OF ZOOLOGY. 



CHAPTER XV. - 

 CTENOPHORA. 



ORDER IV. CTENOPHORA. The Ctenophora comprise "trans- 

 parent, oceanic, gelatinous Actinozoa, swimming by means of 

 ' ctenophoresj or parallel rows of cilia disposed in comb-like plates. 

 No corallum " (Greene). 



The members of this order are all free-swimming organisms, 

 and they are placed by many amongst the Hydrozoa, from 

 which, however, they appear to be clearly separated by the 

 possession of a differentiated digestive sac, as well as by their 

 analogies with the Actinozoa, and their generally superior 

 degree of organisation. 



Pleurobrachia ( Cydippe] may be taken as the type of the order, 



Fig. 89. Adult of Pleurobrachia rkododactyla, in a natural attitude and of the 

 natural size. (After A. Agassiz.) c One of the ctenophores ; t One of the tentacles. 



the structure of all being similar to this in essential points. 

 Pleurobrachia (fig. 89) possesses a transparent, colourless, 

 gelatinous, melon-shaped body, or " actinosoma," in which the 

 two poles of the sphere are termed respectively the "oral" and 



