274 THE HYDRA 



viduals from parts of an original one. If the hydra is cut in two 

 transversely, a new basal portion is formed on the part having 

 tentacles and new tentacles on the basal piece. In a few days 

 two complete hydras are formed which gradually assume normal 

 proportions. These changes may occur even when the hydra is 

 cut into several pieces. 



Other Coelenterata 



Classification. — The Phylum Coelenterata is divided into four 

 classes: the Hydrozoa, which includes the hydra, the forms known 

 as " hydroids," and related jellyfishes; the Scyplwzoa, types 

 of jellyfishes that are usually larger and otherwise differ from the 

 hydroid jellyfishes; the Actinozoa, including the sea anemones 

 and true corals; and the Cte?iophora or sea walnuts, forms which 

 have long been classed as ccelenterates but are regarded by some 

 taxonomists as belonging to a separate phylum. As the life cycle 

 of hydroids presents an unusual mode of development, it will be 

 considered at length, and other representatives of the Coelenterata 

 more briefly. 



The Hydrozoa. — Hydroid Colonies. — With the exception of 

 hydra and a few other genera, none of which are widely distributed, 

 the Coelenterata are marine animals. The Class Hydrozoa includes 

 the hydras, the hydroids which are hydra-like in appearance, and 

 the hydro-medusce or jellyfish that are related to the hydroids. 

 It is characteristic of most hydroids to form colonies, which 

 often contain thousands of individuals, or polyps as they are 

 sometimes called, connected as would be the buds of a hydra, if 

 the latter were to re-bud many times without the individuals 

 becoming detached. Thus, in the genus Obelia (Fig. 133), the 

 units of the colony resemble many individual hydras all con- 

 nected. The individuals have a greater number of tentacles than 

 the hydra and there is a sheath-like skeleton or perisarc. The 

 shape of the hypostome differs, and there are certain other minor 

 differences between the two forms. Yet each polyp resembles a 

 hydra in the fundamental organization of the body into ectoderm 

 and endoderm surrounding a gut cavity, in the radial symmetry, 

 and in the absence of an anal opening. Moreover, the colony 

 arises from a single hydra-like polyp by budding and division of 

 labor among the individuals thus formed (Fig. 133 B). 



I 



