Some of the Lower Metazoans 387 



cavity. At the aboral end the tubular body ends in a basal disc which 

 the animal uses for temporary attachment to objects. Along the sides 

 of the body are often located buds which will give rise to new hydras by 

 asexual reproduction. During periods of sexual reproduction, other 

 projections, the ovaries or testes, may be visible. The various species 

 of Hydra may be either monoecious or dioecious. 



Microscopic Anatomy. — Both longitudinal and cross sections are 

 needed to show the cellular construction of this small metazoan. 

 In a longitudinal section, the animal is seen to be a hollow tube with 



Fig. 127. — Hydra sp. A, Longitudinal section; B, cross section; C, detail of portion 

 of body wall; D, discharged nematocyst. 



the central gastrovascular cavity extending into the tentacles and any 

 buds that may be present. From both types of sections, it is clear that 

 the body wall consists of two cellular layers : the outer epithelium or 

 epidermis and the gastrodermis with the noncellular thin mesoglea be- 

 tween them. The cells of these layers are longer and heavier in the 

 regions of the basal disc and the hypostome. The epithelial cells lining 

 the gastrovascular cavity and those covering the outer surface of the body 

 are of several types. In general, the cells of the gastrodermis are taller 



