VIII. HYDRA 



Hydra is a widely-distributed, fresh-water animal that is par- 

 ticularly well-adapted for consideration at this time since it ex- 

 hibits a number of features which are important as a groundwork 

 for our later study of the much more complex animal types. Thus 

 the sac-like body plan of Hydra, with the enclosed common diges- 

 tive cavity into which the mouth opens, is in line with the condi- 

 tion found in the higher animal types and quite unlike the aberrant 

 condition previously noted in the Sponge in which a digestive 

 cavity of this type is entirely lacking. Hydra exhibits a marked 

 radial symmetry which may be regarded as the primitive, or 

 basic, type from which the two-sided, or bilateral, symmetry of 

 the higher types has arisen. On the whole, the differentiation of 

 groups of cells for various phases of structure and function is 

 probably more advanced in Hydra than in the Sponge. This is 

 particularly true with regard to the nerve cells, or neurons, which 

 really form a primitive nervous system in Hydra. 



A. Structure of Hydra 



In its general structure, Hydra may be compared to a tubular 

 sac, the closed, attached end of which is known as the foot, or 

 basal disc. At the opposite end of the tubular sac, known as the 

 hypostome, there is the mouth opening surrounded by several long, 

 flexible tentacles. The exact number of tentacles varies in the 

 different species of Hydra. The mouth opens directly into a single 

 large central cavity, known as the enteric cavity. The tentacles 

 are hollow structures also, and in each there is a central cavity 

 which is a continuation of the enteric cavity. The body of Hydra 

 is composed of a very large number of cells. These cells are clearly 

 divided into an outer layer, the ectoderm, which covers the entire 

 animal, and an inner layer, the endoderm, which lines the entire 

 enteric cavity even to the tips of the tentacles. Between the 

 ectoderm and endoderm there is a thin layer of transparent, non- 

 cellular material known as the mesogloea. (W. f. 57.) 



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