276 thp: hydra 



tostyles, or non-feeding individuals that produce medusa buds 

 which are set free as (3) the medusae or jellyfish. As shown by 

 Fig. 133, the hydroid colony arises from a single polyp which 

 buds and re-buds until hundreds or even thousands of individuals 

 are formed. These individuals are attached to the vertical stalks 

 that arise from the hydrorhiza or root-like extensions of the 

 mass along the substratum to which it is attached. In this way 

 Obelia colonies often cover the surfaces of submerged woodwork, 

 such as float-stages or the spiles under wharves, and may be 

 collected in masses by removing thin slices of wood from such a 

 surface. 



As the colony approaches maturity, the blastostyles make their 

 appearance, and, on their sides, the medusa buds. The process of 

 forming all these structures, hydranths, blastostyles, and medusa 

 buds, is essentially like that of bud-formation in hydra. The 

 ectoderm and the endoderm grow out, carrjdng with them an exten- 

 sion of the enteron. In the blastostyle no mouth or tentacles are 

 formed. Hydranths and blastostyles never separate from the 

 parent colony, but medusa buds do become detached in a manner 

 that may be compared with what happens to the buds of a hydra. 

 Thus, for purposes of comparison, one may say that Ol^elia is 

 like a colony of hydras formed by budding and by a corresponding 

 division of labor among the units of the colony, such that three 

 types occur: feeding individuals, or hydranths; mouthless indi- 

 viduals, the blastostyles; and sexual individuals, the medusa^, 

 which are set free to swim away and produce male and female 

 germ cells by which sexual reproduction is effected. The produc- 

 tion of the colony with these three types of individuals is obviously 

 comparable with the asexual reproduction in hydra. The medusa, 

 with its reproductive organs, is like a hydra bud set free as a swim- 

 ming individual, and producing either ova or spermatozoa, since 

 the sexes are separate in the medusa? of Obelia. The structure 

 of the medusa is complex (Fig. 127), but its adult structure, as 

 well as its bud-like manner of origin, is homologous with that 

 found in the other units of the colony. It is like a polyp modified 

 for free-swimming life and for its special reproductive functions. 

 It is instructive, after mastering the structure of the medusa, 

 to consider the features of the jellj^fish that are related to its free 

 life, in comparison with the simpler organization of its sister units 

 of the colony, the hydranths. 



