ORGANIC EVOLUTION 



J 57 



THE HYDRA. 



This is a transparent aquatic animal about half an inch 

 long that lives attached to stems and leaves in ponds and 

 sluggish streams. It has a slender tubular body, provided 

 with a disc-like foot at its basal end for attachment, and a 

 circlet of tentacles surrounding a mouth at the other end. 

 Both body and tentacles are very contractile, and become 

 suddenly drawn down into a heap upon disturbance. On 

 this account, although it is immensely larger than the ani- 

 mals studied hitherto, it is difficult to see while collecting in 

 the field. If stems on which it is sought be placed in a 

 shallow white dish of water, or in a glass vessel to be viewed 

 toward the light, it may be seen when it extends itself again 

 after a few minutes, undisturbed. It is likely to be present 

 on loose trash lying in any pond or slow stream, and a good 

 way to find it is to bring in a pailful of this trash and distrib- 

 ute it in aquarium jars to stand over night. If present in 



numbers many of the hydras 

 will move out upon the glass 

 on the lighted side of the jar 

 where they may be readily 

 seen on looking toward the 

 light. Too much trash in a 

 single jar will obscure the 

 view, of course. Specimens 

 may be transferred to a 

 watchglass or slide for study 

 by scraping them loose from 

 their support, and taking 

 them up in a pipette, or bet- 

 ter in a tube attached to a 

 hand bulb. 



In the aquarium hydras 

 may be seen in any position, 



FIG. 100. A hydra. a, extended; b, 

 contracted ; spermary on the body 

 above; o, ovaries below. 



