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 walls perforated by minute pores, and 

 composed of three layers ectoderm, mesoglrea, and endoderm, the 

 last consisting of collared flagellate cells. 



In such an organism as this, imagine the pores to disappear, 

 the internal cavity thus coming to communicate with the exterior 

 by a single terminal aperture ; the mesoglrea 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 around 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 famnar examples of Coelenterata are the horny, 

 seaweed-like " Zoophytes," or, as they are sometimes called, 

 " Corallines," to be picked up on every sea-beach, Jelly-fishes, 

 Sea-anemones, and Corals. The phylum is divided into four classes 

 as follows : 



Class 1. HYDROZOA, including the Fresh-water Polypes, Zoo- 

 phytes, many Jelly-fishes mostly of small size a few Stony 

 Corals, and the peculiar Paleozoic fossils known as Graptolitcs. 



Class 2. SCYPHOZOA, including most of the large Jelly-fishes. 



Class 3. ACTINOZOA, including the Sea-anemones, and the vast 

 majority of Stony Corals. 



Class 4. CTENOPHORA, including certain peculiar Jelly-fishes 

 known as " Comb-jellies." 



CLASS I. HYDROZOA. 



1. EXAMPLE OF THE CLASS OMia. 



General Structure. Obelia is a common zoophyte occurring 

 in the form of a delicate, whitish or light brown, almost fur-like 



