l6 THE HYDRA. 



be long and thread-like, the next it may be all bunched up into a com- 

 pact little mass. Do the filaments surrounding the free end partake 

 of these movements of extension and contraction ? 



STRUCTURAL DETAILS. Study the Hydras in the watch glass, 

 taking note of the following details: 



a) The body, long and cylindrical when extended, short and almost 



globular when contracted. The attached end is usually called the 

 foot. Certain cells at this end secrete a sticky substance which 

 enables the polyp to cling to objects. 



b) Encircling the free end is a group of tentacles. Count them. 



Is the number the same in all your specimens? 



c) Within the circlet of tentacles the body ends in a conical elevation, 



the hypostome, at the rounded apex of which is the mouth. 

 You may not be able to make out this opening unless you have an 

 opportunity to see the polyp feeding. 



d) The mouth opens into an interior cavity called the gastro-vascular 



space, extending the length of the body but having no opening 

 at the lower end. All food is taken in at the mouth and all 

 indigestible matter thrust out at the same opening. 



e) Look for buds growing out from the body at almost any point. If 



you find several on different specimens they will probably show 

 various stages of development. After a time they are constricted 

 off from the parent Hydra and form separate individuals. This 

 is one method of reproduction, the asexual or vegetative. 



f) Hydra has also a sexual method of reproduction. Search for 



ployps showing one or more enlargements on the body wall not 

 far below the tentacles. These are the testes in which are 

 developed a great many sperm cells. Nearer the foot may appear 

 another enlargement, the ovary. In this is developed a single egg 

 cell. The Hydra is, therefore, hermaphroditic, both male and 

 female reproductive organs being present in the same individual. 



g) Note the knotted appearance of the tentacles. This is due to the 



presence of numerous organs of attack and defense, the cnido- 

 blasts. 



