HYDRA 129 



poisonous fluid ; its walls form an ovoid sac, of which the tube is 

 the very much elongated and invaginated outer end. A minute 

 spine projects beyond the free surface of the cnidoblast into the 

 water. When the surface of the ectoderm is irritated the tube is 

 evaginated and shot violently out, and the poisonous fluid con- 

 tained in the cavity of the nematocyst is injected into any animal 

 that may be struck. 



Exercise 2. Make a large semidiagrammatic drawing of the animal 

 showing the details of its structure ; label all carefully. 



Methods of Reproduction. Hydra reproduces both sexually and 

 asexually. Well-fed polyps will soon begin to bud off new indi- 

 viduals. The bud makes its appearance first as a projection of the 

 body wall, and soon becomes a distinct branch. Tentacles and a 

 mouth make their appearance at the extremity of the branch, and 

 the young polyp is complete. It remains attached to the parent 

 for a while ; then detaches itself and begins an independent life. 



Besides this asexual method of reproduction, which may go on 

 as long as the animal is well fed and vigorous, reproduction by 

 sexual methods also occurs at more or less irregular intervals. 

 Sexual organs appear in the form of projections of the ectoderm 

 of the body wall. Two classes of these projections appear : smaller 

 ones, which are testes, and larger ones, which are ovaries. The 

 former of these organs, which lie near the distal end of the animal, 

 produce spermatozoa ; the latter are near the proximal end, and 

 each produces a single large ovum. The green hydra is a her- 

 maphroditic animal, having both testes and ovary when sexually 

 active, whereas the brown hydra is dioecious, being either male or 

 female. 



Exercise 3. Find a polyp with reproductive organs and make a 

 drawing of it. 



Special respiratory, excretory, digestive, and circulatory organs 

 are absent in the hydra. Respiration and excretion are carried on 

 through the surface of the body wall. Digestion, circulation, and 

 absorption go on within the gastro vascular space. The animal's 

 prey is caught in the water with the tentacles, which sting it into 



