142 METAZOAN PHYLA 



victim is added the response to chemicals which may also be produced 

 by it. 



Hydras also exhibit varying physiological states, the reaction of a 

 hungry hydra being distinctly different from that of one which has been 

 fed. When a hydra has fed, it does not seem to be affected by such 

 stimuli as would otherwise cause further food-taking movements and thus 

 does not again feed until the digestion of its food has been completed and 

 the undigested waste passed from the body. 



168. Reproduction. — The hydra reproduces by both sexual and 

 asexual methods, the latter being the one most commonly observed, and 

 the former occurring only at times when conditions of existence become 

 unfavorable. 



Asexual reproduction is most frequently accomplished by budding 

 (Figs. 57 and 58), buds being produced anywhere upon the body. They 

 represent outpocketings of the whole body wall, the cavity of the bud 

 being in direct connection with the enteron. As the bud grows, a mouth 

 appears at its outer end and a ring of tentacles at the base of the hypo- 

 stome. The bud becomes constricted at the point of attachment, 

 separates entirely from the parent, and begins an independent existence. 

 Fission, which is less frequent, is usually longitudinal, although cases of 

 transverse fission occur. 



In sexual reproduction temporary gonads are produced (Fig. 58). 

 The gametes are developed from interstitial cells which accumulate at a 

 certain place, multiply by repeated division, and give rise to oogonia or 

 spermatogonia. In both the spermary and the ovary may be observed 

 all of the steps in a typical gametogenesis. The sperm cells produced are 

 exceedingly numerous and are set free in the water. In the ovary, how- 

 ever, one centrally located egg cell begins early to increase greatly in 

 size at the expense of the other egg cells, feeding upon them and taking 

 them into itself bodily. When this one cell becomes mature it occupies 

 most of the space in the ovary. The ectoderm over it is ruptured, a 

 sperm cell enters, and fertilization occurs. This fertilized egg cell, still 

 in the ovary, undergoes total and equal cleavage. A hollow blastula is 

 formed, which becomes converted into a solid gastrula by the filling in of 

 the blastocoel by entoderm cells derived from the blastoderm. In the 

 meantime a shell has been secreted about the embryo, which now breaks 

 loose from the parent and falls to the bottom. . Further changes involve 

 cellular differentiation and the appearance of the mesoglea. Develop- 

 ment proceeds at a rate varying with different environmental conditions. 

 Finally, when these become favorable, the embryo increases in size and, 

 as it elongates, ruptures the shell. Tentacles appear at one end, an 

 enteron and a mouth are formed by the separation of the cells within, 

 and the young individual gradually assumes the form and characteristics 

 of the adult. 



