. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. M. C. Mereschkowsky on the Hydroida. 249 Fig. 8. must remain intact; for in proportion as the body became elongated, the tentacles lengthened likewise, so that they could always reach the mouth and convey food to it (fig. 7). But if the functions of the tentacles did not undergo any change, we need not expect them to change their form, except perhaps to become longer. This is what we remark in such forms as Cordylojjiioraj Clava, &c., in which the lowest ten- tacles are no
. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. M. C. Mereschkowsky on the Hydroida. 249 Fig. 8. must remain intact; for in proportion as the body became elongated, the tentacles lengthened likewise, so that they could always reach the mouth and convey food to it (fig. 7). But if the functions of the tentacles did not undergo any change, we need not expect them to change their form, except perhaps to become longer. This is what we remark in such forms as Cordylojjiioraj Clava, &c., in which the lowest ten- tacles are not in the least shorter or less developed than the upper ones; on the contrary, they are sometimes a little longer; and in all cases they are all, without exception, longer than the body, owing to which they all have the faculty of assisting in the capture of prey, as has been very well described by Van Beneden in the case of Cordylo^jhora*. I must still mention an articulate type, represented by the genera Tubularia, Acliaradria^ Gorymorpha^ Pennaria^ &c., in which the superior metamere has the tentacles capitate, but much less developed than those of the other me- tamere. All these forms are derived from a non-articulate form with tenta- cles so well developed, so long, and in such great quantity^ that when the formation of the second metamere was induced by some cause, the tentacles belonging to it were perfectly useless to the organism, which caused them to become atrophied, and at the same time capitate—that is to say, adapted solely to the defence of the organism (fig. 8). As the Medusa may be re- garded as a hydranth reversed, and in which the tentacles (radial canals) are united together by a gelatinous substance (ectoderm or bell), it may be understood that the Medusse which have tentacles at the extremity of the manubrium belong to the same articulate type as Tuhu- laria^ Acharadria^ &c., with two metameres, the inferior of which is more strongly marked than
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