FORM OF BODY OF CGELENTERATA. 



93 



Colonies (coruii) 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 off, or it may take place in the stalk- 

 like part. The creeping cormi of the Syncorynida), Hydractinias, 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 (Eudendriuin, Campanularia), or become 

 regularly branched (Sertularia, Pluuiularia) . 



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 gemination 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 form 

 (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-gemmas always carry 

 the organs of reproduction ; from their 

 ova Hydroid Polypes again arise. 

 (Alternation of Generation.) 



While gemmation of Medusas in- 

 tended for a free life distinguishes 

 some Hydroid-Polyps (Fig. 31, a-e, 

 Fig. 32, a-e), in others the process 

 stops at the formation of a Medusa- 

 bud, the organisation of which does 

 not quite attain that stage of develop- 

 ment which conditions the free mode 

 of life, and it remains therefore con- 

 nected with the colony. Nevertheless, 



the normal development of the sexual organs proceeds, and in fact 

 these rudimentary Medusas form " generative buds " (Gonophores), 



Fig. 31. Syncoryne with a num- 

 ber of budding Medusas on it at 

 different stages (a-e) of develop- 

 ment (after Desor). 



