386 The Animal Kingdom 



special shelflike fold of tissue, the velum, around the edge of the bell. Hydra, 

 Obelia, and the jellyfish Gonionemus are examples of this large group. It is 

 only in this class that there are fresh-water representatives. 



Class II. Scyphozoa. In these exclusively marine forms, the hydrold 

 stage is either very small or lacking. The medusa is conspicuous and lacks 

 the velum; the mesoglea has cells. Usually these are free-swimming forms. 

 The large oceanic jellyfish belong to this class; the members of the genus 

 AurelUa are most commonly studied. 



Class III. Anthozoa. The members of this group are exclusively polyps. In 

 them, the mesoglea is a mesenchymal or fibrous connective tissue, the gastro- 

 vascular cavity is divided by numerous septa, and the symmetry is biradial. 

 The corals, sea anemones, sea fans, and sea pens are all members of this 

 entirely marine class. 



THE CLASS HYDROZOA 



Of the coelenterates, those members of the class Hydrozoa are the 

 most varied in both appearance and hal^it. While most of them are 

 marine forms, a few species attain abundance in fresh water. Most 

 species exhibit typical alternation of generations, but some suppress 

 either the hydroid or medusoid form. Of the fresh-water forms, the 

 single species of the genus Craspedacusta has both forms although the 

 medusa is most conspicuous. All other fresh-water forms have only 

 the polypoid stage, suppressing the free-swimming medusoid. The most 

 abundant of the latter group are the hydras which are members of three 

 closely related genera: Chlorohydra which has a green color due to 

 the symbiotic algae living in the cells ; Pelmatohydra which has a stalk ; 

 and Hydra which is neither green nor stalked. Of these, the members 

 of the genus Hydra are most frequently studied. 



Hydra 



These small animals can be seen without the aid of a microscope 

 as tiny colorless individuals attached to the surface water film or 

 perhaps to the glass side of an aquarium. 



External Anatomy. — In size and appearance, hydra varies from 

 a tiny constricted ball to a long slender, almost threadlike form. There 

 is considerable variation in length of individuals, but usually they 

 measure from 10 to 30 mm. The most noticeable feature of the animal 

 is the waving tentacles at the upper or oral end. These hollow ten- 

 tacles surround an opening, the mouth, which is located on a conical 

 elevation, the hypostome. The mouth leads into the gastrovascular 



