HYDRA AND CCELENTERATES IN GENERAL ng 



ment from the parent. In this way a sort of primitive Hydra 

 colony is formed, resembling somewhat the asexual colonies of 

 some of the more complex Ccelenterates to be described later 

 (Fig. 65, A). 



REPRODUCTIVE ORGANS may be observed on specimens of 

 Hydra fusca in September and October. Both an ovary and testes 

 are produced on a single individual ; the former is knob-like, occupy- 

 ing a position about one third the length of the animal above the 

 basal disk; the testes, usually two or more in number, are conical 

 elevations projecting from the distal third of the body. 



Structure (Fig. 54). Hydra is a diploblastic animal con- 

 sisting of two cellular layers, an outer thin colorless layer, the 

 ectoderm (ec.), and an inner layer, the entoderm (en.), twice as 

 thick as the outer and containing the brown bodies which give 

 Hydra fusca its characteristic color. Both layers are composed 

 of epithelial cells. A thin space containing a jelly-like substance, 

 the mesoglea (mes.) separates ectoderm from entodeim. Not 

 only the body wall, but also the tentacles, possess these three 

 definite regions. The body, with the exception of the basal disk, 

 is covered by a thin transparent cuticle. Both body and ten- 

 tacles are hollow, the single central space being known as the 

 gastrovascular cavity (gv.c.). 



ECTODERM. The ectoderm is primarily protective and sensory, 

 containing structures characteristic of these functions. Slight 

 differences in structure are observable between the ectoderm of 

 the tentacles and that of the body wall, while the latter differs 

 from that of the basal disk. In the ectoderm of the body wall 

 are two principal kinds of cells, large epitheliomuscular cells, 

 and small interstitial cells. The latter give rise to cells called 

 cnidoblasts which form nematocysts, and to both male and 

 female germ cells. The epitheliomuscular cells are shaped like 

 inverted cones. At their inner ends are one or more compara- 

 tively long (sometimes .38 mm.) unstriped contractile fibers 

 which form a thin longitudinal muscular layer. These muscle 

 fibers explain the remarkable powers of contraction exhibited 



