An introduction to the study of the comparative anatomy of animals . mes these may produce secondary buds before theydrop off from the parent stem. Now if a Hydra were togrow to a considerable length and produce a single budwhich did not drop off but lengthened out and in turn pro-duced another bud, and if this secondary bud produceda tertiary and the tertiary a quaternary, and so on, a simplecolony of hydriform persons would result, all of whosemembers would be united together and have theirgastrovascular cavities in connection. This condition isrealised in Obelia. There are several species i
An introduction to the study of the comparative anatomy of animals . mes these may produce secondary buds before theydrop off from the parent stem. Now if a Hydra were togrow to a considerable length and produce a single budwhich did not drop off but lengthened out and in turn pro-duced another bud, and if this secondary bud produceda tertiary and the tertiary a quaternary, and so on, a simplecolony of hydriform persons would result, all of whosemembers would be united together and have theirgastrovascular cavities in connection. This condition isrealised in Obelia. There are several species in the genusdiffering from one another chiefly in their mode of budding,and consequently in the eventual form of the plant-likecolonies which result from budding. The species known asObelia geniculata is at once the simplest and most commonof the genus. It is to be found in great abundance just Q 241 Digitized by Microsoft® 242 COMPARATIVE ANATOMY below low-water mark, generally growing on the broad frondsof the oar-weed (Laminaria). A portion of a colony is shown. Fig. 51-A, part of a colony of Obelia geniculata magnified ; hi, hydrotheca contain-ing a hydranth ; gt, a gonotheca enclosing a blastostyle with medusa buds ;/, perisarc; t, terminal growing point. B^ a sexually mature femalemedusa, seen from below ; ?«, mouth ; re, radial canal; cc, circular orring canal; go^ gonads. C, diagrammatic longitudinal section through amedusa; ?«, mouth; 7«^, manubrium; gv^ gastrovascular cavity; rc^radial canal; cc, ring canal; e/, endoderm lamella ; oc, ocellus ; ot^ section is supposed to pass through a radial canal on the left side andan adradial tentacle on the right. Endoderm black; mesoglo^a shaded;ectoderm represented by a broken line. Z>, the bases of two tentaclesmagnified, showing oc, ocelli; ot^ an otocyst on an adradial tentacle; cc,ring canal. in fig. 51, A. The colony consists of a root-like branchingstem, the hydrorhiza, which adheres to the oar-weed and
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