SECTION IV. PHYLUM CGELENTERATA 



IN the previous section we saw that the simplest type 

 of sponge has the general character of a cylinder, closed 

 at one end and open at the other, and having the walls 

 perforated by minute pores, and composed of three layers, 

 ectoderm, mesogloea, and endoderm, the last consisting 

 of collared flagellate cells. 



In such an organism as this, imagine the pores to disap- 

 pear, the internal cavity thus coming to communicate with 

 the exterior by a single terminal aperture ; the mesogloea 

 to be replaced by a very thin, structureless layer containing 

 no cells ; the endoderm cells to lose their collars ; and a 

 circlet of arm-like processes, or tentacles, formed of the 

 same layers as the body-wall, to be developed round the 

 terminal aperture. The result would be a polype, and 

 would serve as a type of the general structure of the group 

 of animals with which we are now concerned. 



The most familiar examples of Coelenterata are the horny, 

 seaweed-like hydroids, or, as they are sometimes called, 

 "zoophytes," to be picked up on every sea- beach, jelly- 

 fishes, sea anemones, and corals. The phylum is divided 

 into four classes as follows : 



Class i. Hydrozoa, including the fresh- water polypes, 

 zoophytes, many jellyfishes, mostly of small size, and a 

 few stony corals. 



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