252 



General Biology 



later becomes the entoderm. The ectoderm then secretes a thick 

 chitinous ( ) shell covered with sharp projections, 



after which the embryo separates from its parent and falls to the bottom 

 of the disk in which it is placed. Here it remains unchanged for several 

 weeks. Interstitial cells then make their appearance followed by an- 

 other resting period, after which the outer chitinous envelope breaks 

 away and the elongation of the escaped embryo continues. Mesoglea 

 is now secreted by the ectoderm and entoderm cells. A circlet of tentacles 

 arises at one end, and a mouth appears in their midst. The young Hydra 

 thus formed soon grows into the adult condition. Almost any part of 

 the Hydra may be cut off, and each part will grow into a complete new 

 animal. This is supposed to be due to the fact that Hydra is an animal 

 composed of tissues which have not yet become organs as in the higher 

 animals. Therefore, the original germ-cells have not divided so often 

 as in higher animals, and each cell contains a little portion of germ- 

 plasm which causes each cell to have the power or potentiality of pro- 

 ducing a complete organism. This theory receives additional weight 

 from the fact that the Hydra can and does reproduce in practically every 

 known way : sexual, asexual, by budding, by longitudinal and transverse 

 fission, in addition to having the ability of restoring any lost part, and 

 of forming a complete new animal from the smallest part. 



When, however, an animal is classified in one of the higher phyla 

 and its somatoplasm is, therefore, further removed from the germplasm, 

 the regenerative ability decreases. This is shown in man, where a piece 

 tet of skin, when removed, will 



be replaced, though an entire 

 finger will not. 



Regeneration means that 

 a part of an organism can 

 reproduce the whole or at 

 least a portion of the lost 

 part. Regeneration is to be 

 distinguished from repro- 

 duction, though in Hydra 

 the two are intimately re- 

 lated. 



As has been stated, there 

 is an alternation of genera- 

 tions in Hydra. The form we have been discussing so far, is called the 

 Hydroid form or the polyp ( . ), while the asexual 



form, so different in appearance from the hydroid, is umbrella shaped 

 and is called a medusa ( ) (Fig. 155). The convex 



portion is usually the upper surface. Tentacles hang down from the 

 edges. At first glance the two forms appear totally dissimilar, but with' 

 a clear conception of what a gastrula really is, we can readily imagine 

 grasping the hydroid form by the mouth and pushing this portion of the 



Fig. 155. Medusa, showing gastrula-form. 



Diagrams showing the similarities of a polyp (A) 

 and a medusa (B). circ, circular canal; ect, ectoderm; 

 end, entoderm; ent. cav, gastrovascular cavity; hyp, 

 hypostome; mnb, manubrium; msgl, mesoglea; mth, 

 mouth; nv, nerve rings; rad, radial canal; v, velum. 

 (From Parker and Haswell.) 



