CHAPTER XXVII 



HYDRA 



A TYPE OF THE PHYLUM COELENTERATA 



Hydras are abundant in bodies of fresh water everywhere and are 

 excluded only from those where the water is foul or the temperature too 

 high. They flourish in those which are clear, cool, and relatively perma- 

 nent. Wherever they occur they may be found attached to soHd objects 

 in the water, such as leaves or stones on the bottom, dead tree branches 



or weed stems, stakes and posts, living vegetation, 

 or the undersurfaces of floating inanimate objects 

 or plants. In any of these situations they will be 

 found extending at right angles to the surfaces to 

 which they are attached and, when hungry, with 

 their bodies and tentacles stretched to the limit. 

 When thus extended, the total length of the body 

 and tentacles may reach two inches or even more. 

 When the object to which it is attached is lifted 

 from the water, the animal contracts and appears 

 like a very small mass of green, brown, or white 

 jelly, depending upon the species under observation. 

 162. External Features. — When examined 

 under a hand lens, a hydra is seen to possess a 

 tubular body which when it is extended is of a 

 practically uniform diameter but which when it 

 is contracted assumes an approximately spherical 

 shape (Fig. 57). The attached end of this 

 Fig. 57. — Hydra viri- body IS known as the hasal disc. The power of 



attachment is due to an adhesive substance pro- 

 duced by gland cells in this disc. The free end 

 of the body bears a ring of tentacles varying in 

 number. Inclosed by these tentacles is a conical 

 projection called a hypostome, at the apex of which is the mouth. Fre- 

 quently one or more huds will be seen projecting from the side of the 

 body, and a bud, if well-developed, may possess its own mouth, hypo- 

 stome, and tentacles. On rare occasions there may be observed on 

 the body of a hydra projections which are temporary reproductive 

 structures. If these are conical and are situated nearer the tentacles 



136 



dissima Pallas. A speci- 

 men possessing a bud, 

 shown in a partially ex- 

 tended, and in b fully- 

 contracted. X about 12. 



