GENERAL APPEARANCE 15 



glass. If the Hydra is detached or fixed to the side of the 

 tank take it up ivith 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 opening to the exterior 



at the mouth, and serving as the digestive cavity of 

 the animal. 



The shape of the body varies very greatly accord- 

 ing to its degree of elongation or contraction. 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 throughout. It is 

 often distended locally by food. 



a. The foot is the closed end of the body which forms 



a kind of sucking disc for attachment to foreign 

 bodies. 



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 part of the body above the 



tentacles. It is conical in shape, 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. 



