II 



CNIDA RIA REP ROD UCTION 



105 



themselves from the mother body, or they may in small numbers 

 remain united with it for some time. In the last case small Hydra 

 colonies composed of similar individuals arise. 



In the same way elegant and richly branched colonies arise in 

 most Hydroids (Fig. 78). The individuals of such stocks are, how- 

 ever, generally not similar, but, as a consequence of more or less 

 division of labour, Dimorphism or Polymorphism takes place. We 

 distinguish : (1) sterile nutritive persons, which remain on the level of 

 the Hydroid, and undertake the feeding of the stock, the gastric cavities 

 of all the individuals of the stock being in communication with one 

 another ; (2) sexual persons, which undertake the duty of ripening the 

 sexual products, and also of planting 

 them out and dispersing them, so that 

 the young brood of Hydroids proceeding 

 from the fertilised egg may attach them- 

 selves in new places and form new stocks. 

 The sexual persons which are destined 

 for a free-swimming life, and which are 

 buds of the Hydroid stock, attain a struc- 

 ture corresponding with this manner 

 of life, they become young Craspedote 

 Medusa, which detach themselves from 

 the stock, swim away, and often after 

 longer or shorter metamorphoses ripen 

 the sexual products. That the Craspe- 

 dote Medusa is only a metamorphosed 

 Hydroid suited for a free -swimming 

 manner of life has already been pointed 

 out. In the accompanying illustrations 

 (Fig. 79, A-E), we can see how a bud 

 of a Hydroid stock develops into a 

 Craspedote Medusa. 



The development of free-swimming 

 sexual persons has also the further 

 advantage that it makes cross-fertilisa- 

 tion possible. 



In many Hydroids, however, there develop on the stock by bud- 

 ding sexual persons whose structure approaches more or less nearly to 

 that of the Medusa, but does not reach it. Such medusoid sexual 

 persons or gonophores (Fig. 80) do not detach themselves as free- 

 swimming Medusa, but develop the sexual products Avhile remaining 

 connected with the stock. It is not possible to define the various sorts 

 of medusoid buds as stages in the development of the Medusa form, as 

 we cannot see what advantage could be gained by the greater or less 

 development of the Medusa form in these attached sexual persons. 

 They should rather be regarded as so many stages of degeneration, 

 due to the fact that the sexual buds no longer detach themselves from 



Fir;. 7;i. .-l, D, C, D, E, Diagram- 

 matic representations of the formation 

 of a Craspedote Medusa by budding 

 from a Hydroid. Black portions = 

 gastric cavities, en, Endoderm ; e, ecto- 

 derm ; mr, gastric peduncle ; v, velum ; 

 ra, radial canal ; r, circumferential canal. 



