ZOOPHYTES 



35 1 



unlike (fig. 869). In the last case they are rounded projections 

 known as sporosacs, a rather unfortunate name, for spores are 

 certainly not produced in them. The ovaries and spermaries of 

 Hydra (see p. 340) may be regarded as sporosacs of the simplest 

 possible kind. The series of egg-producing buds just outlined, 

 from the free-swimming medusa condition down to Hydra, appear 

 to represent a set of evolutionary stages by which the jelly-fish 

 stage as such has been practically superseded. A fixed colony, 

 such as that represented by a 

 Hydroid Zoophyte, is in some 

 respects at a disadvantage, for 

 if markedly unfavourable changes 



Fig. 868. Life-history of Campanularia 



a, Branching base of fixed colony; b, expanded 

 individual ; c, individual retracted into its cup (d) ; 

 e, case within which medusa buds are developing ; 

 ft a free-swimming medusa. 



Fig. 869. Gonophores of various Zoophytes in section 

 diagrammatic 



Digestive cavity black, ectoderm white, endoderm 

 shaded, egg- or sperm-producing cells as small circles. 

 A, Developing gonophore. B-F, Stages in degeneration 

 of gonophores (F is sporosac of Hydra). 



take place in the surroundings it is not able to move away in 

 search of a more desirable locality, and if the area occupied by 

 the species were small, such changes might conceivably lead to 

 its complete extinction, if there were no compensating arrange- 

 ment. Probably the jelly-fish stage was originally evolved as 

 an arrangement of the kind, whereby eggs could be carried to 

 considerable distances, so that a reasonable percentage of the 

 embryos produced from them would have a fair chance of reach- 

 ing suitable spots for settling down and budding into fixed 



