434 ZOOLOGY. 



radially symmetrical with the mouth at one end of the 

 principal axis, but minor features of bilateral symmetry are 

 not uncommon. The body-wall consists of two primary 

 layers of cells, the ectoderm and endoderm, the latter being 

 digestive. These two layers are separated by a layer of 

 gelatinous substance, chemically allied to rnucin and called 

 the mesogloea ; in this layer many cells of various kinds may 

 occur, and these are derived from ectoderm and endoderm. 

 In all Coelenterata except Ctenophora are found nematocysts 

 or thread-cells ; the Ctenophores have no nematocysts, but 

 the surface of the tentacles contains peculiar adhesive cells 

 called colloblasts, which may be equivalent to nematocysts. 

 Ctenophores also differ from other Coelenterata in having 

 a distinct layer of mesoblast cells in the embryo : they are 

 therefore separated by some zoologists as a distinct phylum. 

 They agree, however, with Coelenterates in having no 

 coelom, and may for the present be included in the same 

 phylum. This phylum constitutes a distinct grade of 

 organisation, contrasted with the Coelomata or animals 

 possessing a coelom, a group which includes the remaining 

 phyla. 



As thus denned the Coelenterata are subdivided into 

 Hydrozoa, Scypkozoa, Anthozoa, and Ctenophora. 



CLASS : HYDROZOA. 



Coelenterata in which the coelenteron is simple and the 

 mouth opens directly into it, not into a tube lined by 

 ectoderm or stomodaeum: no mesenteries; reproductive 

 cells discharged directly to the exterior. 



The Hydrozoa as exemplified by Obelia exhibit two dis- 

 tinct forms of individual in the life-history, only one of 

 which is sexual: these are the hydroid and the medusa. 

 The class includes forms like Obelia, in which the zooids 

 of the hydroid stage are supported and protected by cups 

 of the perisarc forming hydrothecae, and other forms in 

 which the perisarc stops short at the base of the zooids and 

 theie are no hydrothecae. The medusae of these two orders 

 differ in structure, those, like Obelia, the Lepto -medusae, 

 having statocysts and the gonads on the radial canals, while 

 the Anthomedusae have no statocysts and the gonads on the 



