120 OUTLINES OF EVOLUTIONARY BIOLOGY 



perisarc. These hydranths are the nutritive individuals of the 

 colony, whose function it is to capture and digest the prey. 



Very often we find, in the angle between a hydranth-bearing 

 branch and the main stem, a short branch bearing an individual 

 or zooid of a different kind, enclosed in a horny cup of totally 

 different shape. These individuals are somewhat club-shaped ; 

 they have no mouth and no -tentacles, and they are termed 

 blastostyles (Fig. 60, A, bst.). The horny cup in which each is 

 enclosed is urn-shaped, with constricted mouth, and is distin- 

 guished by the name gonotheca (gth.). The function of the 

 blastostyle is exclusively reproductive and it is entirely dependent 

 for its nutrition upon digested food received from the hydranths 

 through the hydrocaulus. It reproduces by budding, and the 

 buds are often found attached to it in large numbers, within the 

 sheltering gonotheca, as shown in the figure. 

 ^ The colony itself increases in size by the formation of new 

 buds in regular succession, alternately on the right and left sides, 

 beneath what is, for the time being, the topmost hydranth of each 

 main stem, each new bud giving rise to a hydranth-bearing 

 branch which overtops its immediate predecessor. The buds 

 formed on the blastostyle, on the other hand, do not develop into 

 KyHranths at all, but into another kind of individual known as 

 a .medusa, or medusoid person, which presently detaches itself 

 from the parent and escapes through the mouth of the gonotheca 

 as a free-swimming individual. 



The medusa of Obelia (Fig. 60, B), though larger than the 

 hydranths, is still very small, not more than about T \jth of an 

 inch in diameter. At first sight it looks very different from 

 a hydroid individual (hydranth), but it can easily be shown to 

 have the same fundamental plan of structure. It consists of a 

 circular disk surrounded by a fringe of tentacles (ten.). From the 

 middle of one surface projects a handle-shaped structure, the 

 manubrium (wro.), corresponding to the hypostome of the hydranth 

 and bearing the mouth at its extremity. This mouth leads into 

 a central digestive cavity from which four radial canals (r.c.) run 

 outwards through the gelatinous ground-substance to a circular 

 marginal canal. The medusa swims actively about by muscular 

 contractions of the disk, and in doing so it assumes a bell-like 

 shape with the manubrium projecting from the convex surface, 

 like the handle of the bell. 



Most medusa the larger of which are familiar to us as 



