SUBPHYLUM I : CNIDARIA III 



they are therefore called Diploblastica. The rest of the Metazoa, 

 called collectively Triploblastica, not only have generally bulkier 

 bodies many cells thick, but, after their embryos have reached a 

 stage where the two layers of endoderm and ectoderm are present, 

 they develop a third and usually detached layer between them ; 

 it is called the mesoderm. In place of this the coelenterates have 

 the secreted jelly-like mesogloea, which has little or no structure, 

 although cells sometimes wander into it. Other structural features 

 of the group are the radial symmetry, the nerve-net, and the 

 cavity or enteron in the endoderm with a single opening which 

 serves as both mouth and anus. The first and second of these 

 they share with the echinoderms, in the third they resemble the 

 fiatworms. Except for Hydra and a few other aberrant genera, 

 all coelenterates are marine. 



SUBPHYLUM I— CNIDARIA 



Most of the coelenterates, including both Hydra and Ohelia, 

 belong to this group. They move by muscle-cells, and possess 

 nematocysts. The body is organised on a plan which can be 

 reduced either to a polyp or to a medusa, and often the two forms 

 alternate in one life-history. There is, in fact, little difference, 

 except in shape, between the two, as can be shown by drawing 

 a polyp in section upside down alongside a similar drawing of 

 a medusa (Fig. 75). The larva is typically a planula. There are 

 three classes. 



CLASS I—HYDROZOA 



There is generally an alternation of polyp and medusa, the 

 former being colonial ; the tentacles of the polyp are usually 

 soHd ; there are no vertical partitions in the enteron ; the medusa 

 has a velum and a nerve ring ; the gonads are formed in the 

 ectoderm ; and there is usually an external skeleton. Obelia is a 

 typical member of the class. In some genera, especially those 

 living in the tidal zone (e.g. Tuhidaria), the medusa remains 

 attached to the polyp and the egg develops in situ into a peculiar 

 creeping larva, the actinula. In still other genera the medusa is 

 completely suppressed, and Hydra may be looked on as an extreme 

 example of such suppression, having also other peculiarities, 

 such as the hollow tentacles and sohtary habit. In some 



