140 



ZOOLOGY 



SECT. 



vacuoles of Protozoa, this is the first appearance of specialised 

 excretory organs in the ascending series of animals. 



Besides producing gonads, some medusae multiply asexually by 

 budding, the buds being developed either from the manubrium 

 (Fig. 93, 7a), or from the margin of the umbrella (76). The buds 

 always have the medusa form. 



In many Leptolinae the reproductive zooids undergo a degrada- 

 tion of structure, various stages of the process being found in 

 different species. Almost every gradation is found, from perfect 

 medusae to ovoid pouch-like bodies called sporosacs (Fig. 93, lb t 

 5, s), each consisting of little more than a gonad, but showing an in- 

 dication of its true nature in a prolongation of the digestive cavity 

 of the colony, representing the stomach of the manubrium (Fig. 97). 

 We thus have a reproductive zooid reduced to what is practically 

 a reproductive organ. It is obvious that a continuation of the 



FIG. 97. Diagram illustrating the formation of a sporosac by the degradation of a medusa. A, 

 medusa enclosed in ectodermal envelope (es) ; B, intermediate condition with vestiges of 

 umbrella (u) and radial canals (ra) ; C, Sporosac, ee, ectoderm ; en, endoderm ; m, manubrium ; 

 ov, ovary ; t, tentacle ; v, velum. (From Lang's Comparative Anatomy.) 



same process might result in the production of a simple gonad 

 like that of Hydra : there is, however, no evidence to show that 

 the Fresh-water Polype ever produced medusae, and the probabili- 

 ties are that its ovaries and testes are simply gonads, and not 

 degenerate zooids. The case is interesting as showing how a 

 simple structure may be imitated by the degradation of a com- 

 plex one. It is quite possible, on the other hand, that the 

 reproductive organs of the Leptomedusae (Fig. 88) are sporosacs, 

 i.e. reproductive zooids, not mere gonads. 



In Obelia we found the medusae to be budded off from pecu- 

 liarly modified mouthless zooids the blastostyles. This arrange- 

 ment, however, is by no means universal : the reproductive zooids 

 whether medusae or sporosacs may spring directly from the 

 ccenosarc, as in Bougairivillea (Fig. 92), or from the ordinary 

 hydranths (Fig. 93, 4 an d 5). The primitive sex-cells, from which 

 ova or sperms are ultimately developed, are sometimes formed 



