20 HYDRA. 



over it. If the Hydra is detached or fixed to the side of the 

 tank, take it up with a dipping tube, and place it on a slide 

 in a drop of water, with a small piece of weed for the animal 

 to attach itself to, and to protect it from pressure. Cover as 

 before. 



A. General Appearance. 



Leave the slide until the Hydra has fully expanded, and 

 then examine it with a low power. 



1 . The body is tubular : its cavity, the coelenteron, opens 



to the exterior at the mouth, and serves as the diges- 

 tive cavity of the animal. 



The shape of the body varies very greatly 

 according to the degree of elongation or contrac- 

 tion. It is usually more slender in its proximal or 

 basal half than in the distal portion, but when fully 

 extended is of nearly uniform diameter through- 

 out. It is often distended locally by food. 



a. The foot is the closed proximal end of the body 



which forms a kind of sucking disc for attach- 

 ment to water- weeds, etc. 



b. The mouth is a small aperture at the free or distal 



end of the body. It can be greatly dilated, so 

 as to allow the entrance of food masses of con- 

 siderable size. 



c. The hypostome is the conical part of the body 



above the tentacles, with the mouth at its 

 summit. 



2. The tentacles are hollow processes of the body-wall, 



usually six to eight in number, and arranged in a 

 single whorl round the base of the hypostome, which 

 is the widest part of the body. Their cavities open 

 into the general digestive cavity of the animal. 



The tentacles, which have a warty appearance, 

 are extremely contractile. When fully extended 

 they are two or three times the length of the body ; 

 when fully contracted they are scarcely visible. 



