152 



ZOOLOGY 



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

 110). We thus have a reproductive zooid reduced to what is 

 practically a reproductive organ. It is obvious that a continua- 

 tion of the 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 probabilities 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 

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

 reproductive organs of the Leptomedusae (Fig. 100) are sporosacs, 

 i.e. reproductive zooids, not mere gonads. In some rare cases the 



FiG.ni.— Early development of Eucope. A, blastula^stage ; B, plauula with solid cndoderm ; 

 C, planula with enteric cavity ; al. enteric cavity ; ep. ectoderm ; hy. endoderm. (From 

 Balfour's Bmbryology^ after Kowalevsky.) 



sexual cells are not developed either in medusae or in sporosacs, 

 but are formed directly in the blastostyles. 



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 BougainvilJea (Fig. 104), o"r from the ordinary 

 hydranths (Fig. 105, .^ and 5). The primitive sex-cells, from which 

 ova or sperms are ultimately developed, are sometimes formed 

 from the endoderm or (more usually) ectoderm cells of the repro- 

 ductive zooid; but in many cases originate in the ccenosarc, and 

 slowly migrate to their destination in the ectoderm of the gonad, 

 where they metamorphose in the usual way into the definitive re- 

 productive products, which when mature pass into the space below 

 the ectoderm of the gonad. 



The development of the Leptolinae frequently, but not always, 



