152 



ZOOLOGY 



SECT. 



each consisting of little more than a gonad, but showing an indica- 

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

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

 111). 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 Leptornedusse (Fig. 101) are sporosacs, 

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

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



but are formed 

 directly in the 

 blastostyles. 



In Obelia we 

 found the me- 

 dusae to be bud- 

 ded oft' from 

 peculiarly modi- 

 fied mouthless 

 zooids --the 

 blastostyles. 



FIG. 111. Diagram illustrating the formation of a sporosac by the 1S a r r ^ n g 6- 



degradation of a medusa. A, medusa enclosed in ectodermal ment however 



envelope (<?s) ; B, intermediate condition with vestiges of . ' 



umbrella () and radial canals (ra) ; C, sporosac. cc. ectoderm' IS by no means 



en. eudoderm ; m. mamibrium ; ov. ovary ; f. tentacle ; j>. velum. ,,; ' , l . xi^ 



(From Lang a Comparative Anatomy.) universal . tile 



reproduct ive 



zooids- -whether medusae or sporosacs may spring directly from 

 the ccenosarc, as in Bougainvillea (Fig. 105), or from the ordinary 

 hydranths (Fig. 106, 4 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 coenosarc, and 

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

 where they metamorphose in the usual way into the definitive 

 reproductive products, which when mature pass into the space 

 below the ectoderm of the gonad. 



The development of the Leptolinse frequently, but not always, 

 begins within the maternal tissues, i.e. while the oosperm or im- 

 pregnated egg-cell is stiU contained in the gonad of the medusae or 

 in the sporosac. The oosperm divides into two cells, then into 

 four, eight, sixteen, &c. Fluid accumulates in the interior of the 

 embryo, resulting in the formation of a blastula or hollow globe 



