CLASSIFICATION 249 



into the body with its food. 1 The nematocysts that occur in 

 the Infusorian Epistylis urribellaria and in the Dinoflagellate 

 Polykrikos (p. 131) require reinvestigation, but if it should 

 prove that they are the product of the Protozoa they cannot be 

 regarded as strictly homologous with those of Coelenterata. In 

 many of the Turbellaria, however, and in some of the Nemertine 

 worms, nematocysts occur in the epidermis which appear to be 

 undoubtedly the products of these animals. 



The Coelenterata are divided into three classes : 



1. Hydrozoa. Without stomodaeum and mesenteries. Sexual 

 cells discharged directly to the exterior. 



2. Scyphozoa. Without stomodaeum and mesenteries. Sexual 

 cells discharged into the coelenteric cavity. 



3. Anthozoa = Actinozoa. With stomodaeum and mesen- 

 teries. Sexual cells discharged into the coelenteric cavity. 



The full meaning of the brief statements concerning the 

 structure of the three classes given above cannot be explained 

 until the general anatomy of the classes has been described. It 

 may be stated, however, in this place that many authors believe 

 that structures corresponding with the stomodaeum and mes- 

 enteries of Anthozoa do occur in the Scyphozoa, which they 

 therefore include in the class Anthozoa. 



Among the more familiar animals included in the class 

 Hydrozoa may be mentioned the fresh-water polyp Hydra, the 

 Hydroid zoophytes, many of the smaller Medusae or jelly-fish, 

 the Portuguese Man-of-war (Physalia), and a few of the corals. 



Included in the Scyphozoa are the large jelly-fish found floating 

 on the sea or cast up on the beach on the British shores. 



The Anthozoa include the Sea-anemones, nearly all the Stony 

 Corals, the Sea-fans, the Black Corals, the Dead-men's fingers 

 (Alcyonium), the Sea-pens, and the Precious Coral of commerce. 



CLASS I. HYDEOZOA 



In this Class of Coelenterata two types of body-form may be 

 found. In such a genus as Obelia there is a fixed branching 

 colony of zooids, and each zooid. consists of a simple tubular body- 

 wall composed of the two layers of cells, the ectoderm and the 



1 G. H. Grosvcnor, Proc. Roy. Soc. lxxii. 1903, p. 462. 



