ACTINOZOA. 93 



tons. They agree with the Zoantharia sclerodermata in 

 having a well-developed sclerodermic corallum, but differ 

 from them in the fact that the septa are always some multiple 

 of four / and there are generally transverse plates or tabulae 

 combined with the vertical plates or septa. On the other 

 hand, they agree with the Alcyonaria in having their parts 

 in multiples of four, but differ from them in having a well- 

 developed sclerodermic corrallum in which septa are present. 



ORDER IV. CTENOPHORA (Gr. Jcteis, a comb; phero, I 

 carry). The fourth and last order of the Actinozoa is that of 

 the Ctenophora, comprising a number of free-swimming oceanic 

 creatures, very different in appearance from any of the forms 

 which we have hitherto been considering. They are all trans- 

 parent, 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 trans- 

 verse ridges, whioh are disposed in longitudinal bands, the 

 whole constituting locomotive organs which are known as 

 " ctenophores." In none are there any traces of a corallum or 

 skeleton, and thread-cells are asserted to be universally present. 



FIG. 30. Ctenophora. Pltwrobraclda pileus. 



As the type of the order, we may take one of the commoner 

 forms, which is known by the name of Pleurobrachia or Cy- 

 dippe (Fig. 30). The body of Pleurobrachia is transparent, 

 colorless, gelatinous, and melon-shaped, and exhibits two poles, 

 at one of which is placed the mouth. The globe-like body is 



