132 ANIMAL BIOLOGY 



stimuli the thread may burst out of its capsule and be- 

 come extended with a good deal of force so that it may pene- 

 trate the tissues of animals, even those which are covered 

 with a layer of chitin. The nematocysts contain a pois- 

 onous, irritating fluid which serves to paralyze the small 

 animals that the Hydra fe'eds upon. Nettling cells are 

 commonly furnished with a pointed projection, "the trig- 

 ger/' which is supposed when irritated to set off the dis- 

 charge of the nematocysts. Nematocysts are especially 

 abundant near the tips of the tentacles where they are 

 most apt to come into contact with prey. The tentacles 

 are also adhesive and small animals which come in contact 

 with them are caught and drawn toward the mouth. 

 The swallowing capacity of Hydra is enormous; animals 

 considerably larger than the Hydra itself are successfully 

 taken into the digestive cavity. 



Reproduction in Hydra is effected in two ways, (i) 

 asexually, by the formation of buds, and (2) sexually, 

 through the production of eggs and sperms. In budding, 

 an outpushing of both layers of the body wall occurs, 

 tentacles are pushed out and a mouth breaks through at 

 the outer end of the bud. Finally the bud constricts off 

 at the base and forms a new free Hydra. Often many 

 buds are found on one individual. 



Both male and female sex cells are produced in the same 

 individual. The eggs are produced, one or two at a time, 

 on the basal part of the body. The sperms appear in a 

 number of conical prominences nearer the oral end. The 

 sperms are set free in the water and fertilize the large egg 

 cell while it is still in the ectoderm of the body wall. In 

 this situation also the egg undergoes its early develop- 

 ment, but at a certain stage there is formed around it a 

 chitinous and often spiny shell, or capsule, which serves 

 to protect the egg after it is discharged, when it comes to 



