THE COELENTERATA 251 



; ! 



tion bud off and make a sort of solid gastrula-like structure; this later 

 becomes the entoderm. The "Ectoderm now secretes a thick chitinous 

 ( ) shell covered with sharp projections. The embryo 



then separates from its parent and falls to the bottom, where it remains 

 unchanged for several weeks. Then interstitial cells make their appear- 

 ance. A subsequent resting period is followed by the breaking away of 

 the outer chitinous envelope, and the elongation of the escaped embryo. 

 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 com- 

 plete 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 

 as 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 

 producing 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 



of skin will grow again 

 when removed, though an 

 entire finger will not be re- 

 placed. 



Regeneration means that 

 a part of an organism can 

 reproduce the whole or at 

 least a portion of the lost 

 part. This is distinguished 



Fig. 155. Medusa, showing gastrula-form. f rom reproduction, though 



Diagrams showing the similarities of a polyp (A) : n ITtrHt-a +Vio +,,,~ ~~, 



and a medusa (B). circ, circular canal; ect, ectoderm; in ^V^a the tWO are mtl- 



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

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



mouth ; nv, nerve rings ; rad, radial canal ; v, velum. As has been Stated there 



(From Parker and Has well). 



is an alternation of genera- 

 tions in this animal. 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 Called a medusa ( ), (Fig. 155.) The convex 



portion is usually the upper surface, and tentacles hang down from the 

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

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



