. Elements of Comparative Anatomy. FOEM OF BODY OF CCELENTERATA. 93 Colonies (corml) are formed from single animals by gemmation. This may either occur at any point of the surface of the body (Hydra), and end by the bud breaking oif, or it may take place in the stalk- like part. The creeping cormi of the Syncorynidac, Hydractini^, etc., are formed by processes of the basal part, which give off new animals, attached here and there to it. When gemmation occurs in the free part of the stalk we get free, branched colonies, which become compli- cated in very various ways (Eudendrium, Campanularia),
. Elements of Comparative Anatomy. FOEM OF BODY OF CCELENTERATA. 93 Colonies (corml) are formed from single animals by gemmation. This may either occur at any point of the surface of the body (Hydra), and end by the bud breaking oif, or it may take place in the stalk- like part. The creeping cormi of the Syncorynidac, Hydractini^, etc., are formed by processes of the basal part, which give off new animals, attached here and there to it. When gemmation occurs in the free part of the stalk we get free, branched colonies, which become compli- cated in very various ways (Eudendrium, Campanularia), or become regularly branched (Sertularia, Plumularia). The formation of colonies is almost always accompanied by the formation of a tubular investment, which is a secretion from the surface of the body, and which serves as a support for the whole trunk, as well as for its branches; it is also continued, in various degrees, on to the persons of the colony. § 74. The process of gemmation in the Hydroid Polypes produces, in addition to the growth of the colony by freshly-formed similar individuals (persons), structures of quite a different kind, the most, differentiated forms of which are developed into Medusas. The body of these buds is of a bell-shaped or discoid foi'm (Fig. 32, m), and by its internal organisation, as well as by the tentacles, which arise from the edge of the bell, or disc, we are able to make out secondary axes, generally two in number, which cross the primary axis at right angles to one another, and are completely equivalent one with the other. A higher grade than that of the Hydroid-Polyps is expressed in this organisation. The animals move by contractions of the bell, the edge of which is produced into a membrane, the velum, which is also contractile. These Medusa-gemm^ always carry the organs of reproduction; from their ova Hydroid Polypes again arise. (Alternation of Generation.) While gemmation of Medusae in- tended for a free life distinguishes some H
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