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, having the walls per- 

 forated 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 dis- 

 appear, the internal cavity thus coming to communicate 

 with the exterior by a single terminal aperture ; the meso- 

 glcea to be replaced by a very thin structureless layer con- 

 taining 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 " Zoophytes," or, as they are sometimes called, 

 " corallines," to be picked up on every sea-beach, the 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 Jelly-fishes mostly of small size and a 

 few Stony Corals. 



