CHAPTER XV 

 ON CLASSIFICATION 



HYDRA and Obelia both belong to the same class, Hydro- 

 medusas, of the phylum Coelenterata. A coelenterate animal is 

 radially symmetrical with a mouth opening into a single gastro- 

 vascular cavity serving alike for digestion and circulation ; its 

 body wall is formed by the two primary cell layers, ectoderm 

 and endoderm, and between these is a layer, more or less 

 thick, of a jelly-like substance, the mesoglcea. In the Hydro- 

 medusae the mouth opens directly into the gastrovascular 

 cavity, and the latter is not complicated by the presence of 

 partitions, ridges, or filaments in its interior. The other classes 

 of the Coelenterata are the Acalephse, the Anthozoa, and the 

 Ctenophora. The large jelly-fishes which abound in our seas 

 in the warm months belong to the Acalephse. In this class 

 the medusa is the dominant phase in the life-history of the 

 organism, and the gastrovascular cavity is furnished with gastral 

 ridges or gastral filaments. The Anthozoa comprise the sea- 

 anemones and corals. In them the mouth does not open directly 

 into the gastrovascular cavity, but into a short tube lined by 

 ectoderm which hangs down in and opens below into it, The 

 gastrovascular cavity is further complicated by the presence 

 of a number of radial partitions called mesenteries passing 

 from its walls very nearly to its centre. The Ctenophora are 

 transparent marine animals which swim through the water by 

 the united action of a number of comb-like plates of cilia. 

 In structure and. development they differ a good deal from 

 other Coelenterata, and must be regarded as an aberrant offshoot 

 of the phylum. Amongst other peculiarities they do not 

 possess nematocysts, but are provided instead with peculiar 

 adhesive structures known as lasso-cells. 



In the accepted system of classification a phylum is divided 

 into classes ; a class into orders ; an order into sub-orders, and 



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