HYDRA AND CCELENTERATES IN GENERAL 



capable of regeneration; for example, the skin, muscles, nerves, 

 blood vessels, and bones. Lost parts are not restored in man, 

 because the growing tissues do not coordinate properly. Many 

 theories have been advanced to explain regenerative processes, 

 but none has gained sufficient acceptance to warrant its inclusion 

 here. 



2. CCELENTERATES IN GENERAL 



The Characteristics and Classification of Ccelenterates. - 



The Phylum Ccelenterata includes the polyps, jellyfishes, sea 

 anemones, and corals. All of these animals have a body wall 

 consisting of two layers of cells, between which is a non-cellular 

 substance, the mesoglea. Within the body is a single gastro- 

 vascular cavity, or ccelenteron. Because of the presence of two 

 cellular layers, all Ccelenterates are said to be diploblastic. They 

 are also acoelomates, i.e. they do not possess a second body cavity, 

 the ccelom. All Ccelenterates are provided with nematocysts. 



This phylum contains three classes, as follows : 



Class i, Hydrozoa. This class includes the fresh-water polyps, 

 the small jellyfishes, the hydroid zoophytes, and a few stony 

 corals. 



Class 2, Scyphozoa. Most of the large jellyfishes are placed 

 in this class. 



. Class 3, Anthozoa. In this class are found the sea anemones, 

 and most of the stony and horny corals. 



Hydra is a member of the class Hydrozoa. It may be considered 

 as a type of what is known as a zooid, or polyp. Many of the 

 Hydrozoa living in salt water are colonial, consisting of a large 

 number of zooids which are united so as to resemble a branching 

 tree, e.g. Obelia (Fig. 65, A). If the buds of a Hydra should 

 remain attached to their parent, and should in turn produce buds 

 a hydroid colony somewhat like Obelia would result. All Hydro- 

 zoa are not fixed, but some of them swim about freely through the 

 water. The jellyfishes, or medusce (Fig. 65, B), are of this type. 

 They are cup-shaped animals with a circlet of tentacles extending 



