270 THE HYDRA 



coplenterates, like the jellyfish, which are more active animals 

 than hydra, a similar primitive centralization of nerve cells has 

 produced a definite nerve ring about the margin of the bell, and 

 a similar development in the hypostome. 



There remain the cnidoblasts (Fig. 129), which do not seem 

 to be connected with the nervous system, although it is impos- 

 sible to say that there are no such connections. Judged by 

 its structural relationships and manner of functioning, the cnido- 

 blast appears to be an independent effector, that is, a cell capable 

 of direct response to stimuli. In the comparison of the nervous 

 systems of various animals, in Chapter 13, it will be shown 

 that such effectors probably represent the beginnings of the 

 sensory-neuro-muscular mechanism of animals. 



Reproduction. — Asexual Reproduction. — Hydra frequently 

 produces new individuals by budding. As there is nothing 

 involved in this process that can be identified as maleness or 

 femaleness, it may be properly termed "asexual" (c/. p. 219). 

 There first occurs an accumulation of nutrient material in the 

 endoderm cells at some place toward the middle of the body and 

 the interstitial cells in this region increase in numbers. Gradu- 

 ally a bud-like swelling develops, into which, at an early stage, 

 there grows an extension of the enteron so that the bud becomes 

 a blind outgrowth of the two layers of the body wall (Fig. 121). 

 As the development proceeds, tentacles begin to appear as lesser 

 outpushings of ectoderm and endoderm, and finally the mouth is 

 formed. Usually, the bud closes its connection with the parent's 

 enteron about the time the tentacles appear, and soon after 

 pinches itself off, leaving no permanent scar at its place of origin. It 

 thus drops to the bottom and becomes a new and independent 

 individual. In some cases, however, the bud remains longer on the 

 parent and may even re-bud so that there may be several genera- 

 tions in a branching system. This is comparable with the colonial 

 state in hydroids (cf. p. 274), where the budding is so extensive 

 that colonies are produced, containing hundreds of individuals 

 originating from a single original hydra-like organism (cf. Fig. 134). 



Sexual Reproduction — At certain seasons of the year, particu- 

 larly in the fall, hydra reproduces sexually by means of male and 

 female cells. The testes are usually located on the upper half 

 of the body. The ovaries lie near the middle of the body. Testes 

 may appear first and ovaries later, on the same animal, or both 



