THE HYDROMEDUSAE 27 



fication of the gonophore, the reduction of the high organisation of 

 the free-swimming medusoid, is as noticeable in the Leptomedusae 

 as in the Anthomedusae. 



1. The gonophore retains certain medusoid structures, such as 

 the radial and circular canals and the tentacles, but the mouth 

 is never perforated, and the organism never freed (Gonothyraea). 

 In this, and in many far simpler gonophores of the Leptomedusae, 

 the ectoderm of the blastostyle is multilaminar ; the inner 

 layer gives rise to the entocodon and the exumbral ectoderm 

 of the gonophore ; the outer layers, separated from the inner by 

 a space, form a more or less complete sheath of the gonophores, and 

 appear also as irregular strands (gubernaculum, Fig. 36) between the 

 blastostyle and the perisarc of the gonotheca. 2. The gonophore 

 develops entocodon, manubrium, and radial canals, but is devoid 

 of velum and tentacles (1 Laomedea repens). 3. The gonophore 

 develops no entocodon ; the ectoderm is multilaminate ; traces of 

 an endoderm lamella are indicated (female Sertularia pumila), or 

 are entirely absent (Aglaophenia). 4. No trace of the gonophore 

 remains ; the generative cells are borne directly on the blastostyle 

 (male Sertularella polyzonias). 



In these simplified gonophores the generative cells frequently 

 lie in the manubrium, as in the Anthomedusae, but not in the 

 position of the radial canals, as is the case with the free-swimming 

 medusoids of this group. The gonophore or the blastostyle may 

 grow directly from the hydrocaulus (Campanulina) ; most com- 

 monly they arise from the axil between a hydrotheca and the 

 hydrocaulus or hydrocladium (Halecium), or in the centre of a 

 hydrocladium (Aglaophenia). They may spring directly from the 

 hydrorhiza (Coppinia). 



THE GONOTHECA AND PHYLACTOCARP. In most cases the 

 gonophore, or the blastostyle and gonophore, are enclosed in a 

 rigid, horny capsule, continuous with the perisarc, and termed the 

 gonotheca (Fig. 35). This is generally oval, or shaped like a flask or 

 an amphora ; and is either smooth (Obelia), spinose (Plumularia), or 

 annulated (Campanularia). It encloses either a medusoid (Cam- 

 panulina), or a blastostyle carrying medusoids (Obelia), or a 

 sporosac (male Halecium), or a blastostyle carrying sporosacs 

 (male Plumularia). It is frequently provided with a hinged oper- 

 culum (Plumularia). 



The modifications connected with the gonophore and gonotheca 

 vary greatly in different genera and species, and even in different 

 sexes of the same species ; only a few of their types can be sketched 

 here. 



1. In Gonothyraea, as already mentioned, a hardly disguised 

 medusoid is developed ; each gonophore, when nearly mature, 

 migrates up the blastostyle and projects beyond the mouth of the 



