8 4 



MANUAL OF ZOOLOGY. 



central cavity 



exist in the following forms: i. As "sporosacs," or simple 

 closed sacs, consisting of ectoderm and endoderm, with a 

 in which ova and spermatozoa are produced. 

 2. As " disguised medusoids/' in 

 which there is a central manubrial 

 process and a rudimentary system 

 of gonocalycine canals ; but the 

 gonocalyx remains closed. 3. As 

 complete medusoids, which have a 

 central manubrium, a complete sys- 

 tem of gonocalycine canals, and an 

 open gonocalyx ; but which never 

 become detached. 4. As perfect 

 medusiform gonophores (fig. 15), 

 which are detached, and lead an 

 independent existence for a time, 

 until the generative elements are 

 matured. In whichever of these 

 forms the gonophore may be pre- 

 sent, the place of its origin from the 

 trophosome may vary in different 

 species of the order. T. They may 

 arise from the sides of the polypites, 

 as in Coryne and Stauridia. 2. 

 They may be produced from the 

 ccenosarc, as in Cordylophora. 3. 

 They may be produced upon cer- 

 tain special processes, which are 

 termed "gonoblastidia," as in Hy- 

 dr actinia and Dicoryne. These 

 gonoblastidia are processes from the body-wall or ccenosarc, 

 which closely resemble true polypites in form, but differ from 

 them in being usually devoid of a mouth, and in having 

 shorter tentacles. 



As regards the development of the Corynida, the embryo is 

 very generally, though not always, ciliated at first, and be- 

 comes developed into a hydra-form polypite, which fixes itself 

 to some foreign body, and then (if not belonging to one of the 

 simple forms) proceeds to produce by gemmation the com- 

 posite adult. The development of the Corynida (as well as 

 that of the Sertularida and Lucernarida) obeys the general law, 

 that the new polypites are developed at, or near, the distal end 

 of the hydrosoma ; so that the distal polypites are the youngest, 

 the reverse of this obtaining amongst the oceanic Hydrozoa. 

 The subject of the reproduction of the Corynida having 



Fig. 15. Free medusiform gono- 

 phore of Clytia Johnstoni (after 

 Hincks). a Central polypite or 

 manubrium ; b b Radiating gastro- 

 vascular canals ; c Circular canal ; 

 in Marginal bodies ; t Tentacles. 



