COELENTERATA AND CTENOPHORA 



Monoeciousness may be the usual condition in hydras, ahhough species in 

 which the individuals seem to be exclusively male or female, hence dioecious, 

 have been reported. There are no secondary sexual characteristics in hydras; 

 only by observing the testes or ovaries can the sex of an individual be 

 determined. These gonads appear as swollen protuberances from the epi- 

 dermis in the characteristic regions (Fig. 10.7). Within them, ova or sperma- 

 tozoa arise from interstitial cells. Fully matured spermatozoa may be seen 

 moving actively within the testis; they are discharged by the periodic opening 

 of the apex of the testis, which thus liberates successive swarms. The sper- 

 matozoon then swims about until it dies, or until it comes into contact with 

 an ovum which has been exposed by the rupture of its epidermal covering. 

 The zygote formed by the union of these two gametes undergoes cleavage and 

 secretes about itself a shell-like cyst, or theca. Within the theca, development 

 proceeds until an outer layer of cells, the ectoderm, and an inner solid mass, 

 the endoderm, have been formed. The embryo within its theca then becomes 

 detached from the parent and drops to the bottom. Tentacles eventually 

 develop; the embryo breaks from its cyst, becomes attached, develops a 

 coelenteron, forms a mouth, and so becomes a miniature hydra. Zygotes 

 developing in late autumn pass the winter within the protective cyst. 



Hydras frequently produce new individuals by budding, a process referred 

 to as asexual reproduction (Fig. 10.8). It is, essentially, reproduction by 

 cell division. It differs, however, from the asexual reproduction of protozoans 

 in that the mass of new cells produced is organized by some integrating in- 

 fluence into a multicellular individual with the characteristics of the parent. 

 There is first an accumulation of nutrient material in the gastrodermal cells 

 at some place toward the middle of the body, and cells in the epidermis of 

 this region divide repeatedly to form a bud-like swelling. An extension of 

 the coelenteron grows into the bud, which then appears as a blindly ending 

 outgrowth of the two layers of the body wall. Tentacles appear as evagina- 

 tions of epidermis and gastrodermis, and finally a mouth is formed. If food 

 is abundant, the bud may remain attached to the parent for some time, and in 

 exceptional cases it may rebud to form several generations in a branching 

 system. Usually, however, the connection between parent and off^spring be- 

 comes constricted, and the bud is detached as an independent individual as 

 soon as the tentacles and mouth become functional. 



Growth and Regeneration. Recent research has demonstrated the interest- 

 ing fact that a region just below the hypostome of the hydra constitutes a 

 zone of proliferation. In this area new cells are constantly being pro- 

 duced by mitotic activity and added to the layers making up the body wall. 

 There is a constant slow progression of cells downward toward the base; the 

 oldest cells in the body make up the base itself and are gradually sloughed 

 ofT at this point. Thus, the base and the portions of the stalk adjacent to 

 it are composed of aged, "exhausted" cells, whereas a continuous cvcle of 

 replacement provides young, vigorous cells in the more distal regions. 



Like many other types of animals with simple organization and well- 



299 



