STRUCTURE AND FUNCTION 55 



Definitions. 



Proximal end, the end by which an animal is attached to 

 an object. 



Distal end, the end opposite the proximal end. 



Tentacles, slender projections around the distal end. 



Mouth, the opening through the distal end, into the cen- 

 tral cavity. 



Bud, a small hydra or other ccelenterate growing out 

 from the wall of the parent. 



Mesoglea, a thin, gluey partition, without wandering cells, 

 between the ectoderm and the endoderm. 



Nettle cells, very small cells, chiefly in the tentacles, 

 easily identified in permanent preparations as clear cells 

 with small hairs projecting from them. See text-books 

 for details of their structure. 



Spermary, the region or organ where the sperm cells are 

 formed. 



Ovary, the region or organ where the egg cells are formed. 



Ccelenterates (hollow bowels), sac-shaped animals, the 

 digestive tract having only one opening ; the body wall is 

 of two layers. 



Directions. 



Take a small aquarium to your table, set it down care- 

 fully and leave it undisturbed. Identify a hydra and 

 watch it for some time. 



Observations on the living animals. 



I . Describe the size and shape of a hydra when expanded. 

 Disturb it slightly by shaking the aquarium a little, and de- 

 scribe its shape when contracted. Notice also the flexi- 

 bility of the body. What do you infer concerning the 

 hydra's possession of a skeleton .? What advantage can it 

 be to have a body so flexible .' 



