ACTINOZOA. 95 



Order IV. Ctenophora. 



The fourth and last order of the Actinozoa is that of the Ctenophora, 

 comprising a number of free-swimming oceanic creatures, very dif- 

 ferent in appearance from any of the forms which we have hitherto 

 been considering, and often placed among the llydrozod. They aie 

 all transparent, gelatinous, glassy-looking creatures, which are found 

 near the surface in the open ocean, swimming rapidly by means of 

 bands of cilia. The cilia are arranged in a series of transverse ridges, 

 which are disposed in longitudinal bauds, the whole constituting loco- 



Fig. 53.— Adult of PleurobracMa rliododactyla, in a natural attitude and of the natural 

 size. (After A. Agassiz.) c One of the ciliated bands or otenophores ; t One of the 

 tentacles. 



motive organs which are known as " ctenophores " (fig. 58, c). In 

 none are there any traces of a corallum or skeleton, and thread-cells, 

 or cells which represent these, are generally present. As the type 

 of the order, we may take Pleurohrachia or Cydippe (fig. 58). The 

 body of Pleurohrachia is transparent, colourless, gelatinous, and 

 melon-shaped, and exhibits two poles, at one of which is placed the 

 mouth. The globe-like body is divided into a number of crescentic 

 lobes by eight ciliated bands or ctenophores, which proceed from 

 near the mouth to near the opposite pole of the body. Besides the 



