SECTION IV. — PHYLUM CCELENTERATA 



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, mesoglcea, 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 mesoglcea 

 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 Ccelenterata 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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