COELENTERATA AND CTENOPHORA 



it is often seen attached to the surface film, hanging down into the water. 

 Although there are no special organs of locomotion, the animal shifts its 

 position by simple movements of the body or by a slow gliding of the base 

 without detachment from the substratum. Hydras react positively to light of 

 moderate intensity and so tend to collect in the lighted portion of an 

 aquarium. This reaction is cspeciallv noticeable in the green hydra. But let 

 us defer further consideration of habits and general behavior until the struc- 

 tures involved have been described. 



General Structure. The body of a hydra consists of a simple two-layered 

 tube (Fig. 10.2), the trunk, normally attached at one end, the base, and 

 surmounted at the other by a circle of tentacles, varying in number. The 

 tentacles enclose a conical region, the hypostome, which bears at its apex the 

 mouth. The body wall surrounds a digestive cavity or coelenteron, which 

 extends into the tentacles. The cell layers, an outer epidermis and an inner 

 gastrodermis, are separated from each other by a non-cellular supporting la- 

 mella. This structure corresponds functionally to an elastic skeleton. It 

 serves as a place of attachment for the cells and gives support and elasticity 



Secretory cells 



Supporting lamella 

 Gastrodermis 



Tentacle 



Nematocyst 



Bud 



Fig. 10.2. Hydra: general structure, shown by a diagrammatic longitudinal section of the body- 



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