THE SPONGES AND THE TWO-LAYERED ANIMALS 



ployed as a tutor for a distinguished French 

 family where one of his duties was to teach 

 natural history to the young men of the 

 family. He entertained these boys by 

 cutting hydras, as well as other coelenter- 

 ates, into small pieces and watching them 

 regenerate into adult forms (Fig. 8-9). 



By splitting the hypostome region Trem- 

 bley was able to obtain a double-headed 

 animal. He also found that by passing a 

 needle with a knotted thread throuo"h the 

 mouth and the basal disc, he could com- 

 pletely invert the two layers; in other 

 words, he turned the hydra inside out. 

 Then, instead of inverting; back to its origi- 

 nal condition, the animal simply moved its 

 epidermal cells in between the endodermal 

 cells so that both layers of cells found 

 their original location. Since Trembley's 

 time, a great deal of work has been done on 

 regeneration, not only of hydra but also of 

 many other animals. In general, if pieces of 

 hydras are grafted together in various posi- 

 tions, the parts retain the characteristics 

 which they originally possessed. It has been 

 shown, for example, if the mouth region of 

 one animal is grafted to the basal disc of 

 another, the development of tentacles 

 is induced. In other words, an anterior end 

 develops so that the resulting animal has 

 two "heads" and no "foot." 



An understanding of regeneration among 

 these simple Metazoa has considerable 

 significance. Experiments demonstrate that, 

 as the animal scale is ascended from simple 

 to complex, this ability to replace lost parts 

 or regenerate whole bodies is gradually 

 lost. Hydra can replace its entire body from 

 a fragment. In animals slightly more com- 

 plex than hydra this ability is confined to 

 the replacement of a part. Finally in ani- 

 mals as complex as man all power of re- 

 generating parts has been lost, and the only 

 remnant of this endowment left is the 

 ability to heal or close over a wound. The 

 significance of this comes closer when it is 

 realized that this is the basis of all plastic 

 surgery, which is playing a more and more 



147 



Fig. 8-11. A hydra with many testes. The fourth testis 

 from the anterior end is nearly mature. Note the 

 nipple-like tip from which the sperm will be dis- 

 charged. This is a stained specimen. 



significant role in the lives of people where 

 exposure to serious injury is so common. 



Hydra also reproduces sexually by the 

 production of eggs and sperms (Fig. 8-10). 

 Usually both ovaries and testes are formed 



