CNIDARIA ' 43 



of the body -wall. Finally, the perisarc is ruptured at this 

 place, and the polyp becomes fully expanded (Fig. 15 F). 



The origin of the polyp stock does not always take place exactly accord- 

 ing to the plan here given. In certain cases the larva becomes attached 

 by its side, and is almost wholly employed in the formation of the hydro- 

 rhiza, while the first hydranth grows out of it by a kind of budding 

 (Mitrocoma, Metschnikoff). 



The hydroid polyp thus produced propagates principally 

 by means of lateral budding. There is formed at first a 

 hernia-like protrusion of the body-wall, the cavity of which 

 communicates with the gastral cavity of the parent animal, 

 and the wall of which consists of the same layers as that of 

 the parent (ectoderm, entoderm, and the sustentative lamella 

 between them).^ The bud is converted into a hydranth by 

 the progressive abstriction of this protrusion from the parent 

 animal, by the production of a crown of tentacles, and by the 

 acquisition of a mouth-opening through dehiscence at the 

 anterior end. Only rarely do the hydranths thus produced 

 detach themselves from the parent, and become independent 

 (Hydra) ; in most cases extensive polyp colonies are formed 

 by continued budding. The laws of budding, by which the 

 extraordinarily manifold shape as well as habit of the polyp 

 colonies is determined, have recently been subjected to a 

 critical study by Weismann (No. 49) and by H. Driesch 

 {Inaug.-Diss., Jena, 1889). 



The individuals produced by budding do not always have the same 

 form as that of the first hydranth. A more or less pronounced poly- 

 morphism often arises among the individuals of a colony. There are 

 produced defensive polyps (spiral zooids), abundantly supplied with 

 nettle-capsules, but destitute of tentacles and oral opening, hard-shelled 

 spiny protective polyps, nematophores, etc. The so-called blastostyle, 

 which occurs in many Hydroids, is also to be interpreted as a meta- 

 morphosed, non-tentacular polyp, upon the lateral walls of which the 

 gonophores are produced by budding. 



The formation of the individuals destined to become 

 sexually mature (medusas, sessile medusoid buds) is the 



' [Recently Alb. Lang (No. IX., Appendix to Literature on Cnidaria) 

 has maintained that the whole of the bud in Hydroids is derived from 

 the ectoderm of the parent.] 



