SECTION IV 

 PHYLUM CCELENTERATA 



The possession of an interf^al cavity lined by a special internal 

 layer of cells — the endoderm — in which the digestive and absorp- 

 tive functions are centred, distinguishes all the remaining groups 

 of Metazoa from the Parazoa or Sponges. The former are grouped 

 together under the comprehensive title of Enterozoa, or animals 

 with enteric cavity. The simplest Enterozoa have an internal 

 cavity in which there is no separation between the enteric 

 or digestive cavity and the coelome or body-cavity — one con- 

 tinuous space representing both and opening on the exterior by 

 the aperture of the mouth. These constitute the phylum 

 Calenterata. They are all animals of a low type of organisation 

 with a conspicuous radial symmetry, disguising, in some cases, 

 a more obscure bilateral arrangement, which may be more 

 primitive. 



The most familiar 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 Palseozoic fossils known as Grapiolites. 



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



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

 majority of Stony Corals. 



Glass 4. Ctenophora, including certain peculiar Jelly-fishes 

 known as "Comb-jellies." 



CLASS I.— HYDROZOA. 



1. Example of the Class — Obelia. 

 General Structure. — Obelia is a common zoophyte occurring 

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



