HTDROZOA SCYPHOMEDUS.i;. 255 



of ocular and auditory apparatus. The auditory function is provided 

 for by a large sac containing crystals, which originates from the cells 

 of the eiitoderm ; while the eye consists of a mass of pigment lying on 

 the dorsal or ventral face, and nearer the end of the stalk. In some 

 exceptional cases (Nausithoe) it is provided with a refractile cuticular 

 lens. But it is in the Charybdeida- that the sense body reaches the 

 highest development ; for in them, in addition to the terminal :-ac 

 of otoliths, there is also present, in the wall of the dilated vascular 

 space of the papilla, an extremely complicated visual organ, formed 

 of four small paired and two large unpaired eyes, in which lens, 

 vitreous body, and retina can be distinguished. 



The four generative organs of the Acalepha can be easily dis- 

 tinguished in consecpaence of their size and their bright colouring. 

 In some cases, at any rate in the Discopkora, they protrude as folded 

 bands into special cavities in the umbrella, the so-called sub-genital 

 pits (hence the term Pkanerocarpce Esch.) In all cases these bands 

 lie on the lower (sub-umbrella) wall of the digestive cavity (figs. 194, 

 195), from which they originate as leaf -like prominences. The 

 upper surface is covered with gastric epithelium ; the under, which 

 is turned towards the sub-umbrella, with germinal epithelium, the 

 elements of which, in the process of development, pass into the 

 gelatinous substance of the band. 



The formation of the cavities in the sub-umbrella of the Discophora 

 is due to a local growth of the gelatinous substance of the sub -umbrella; 

 in some cases, however, they may be completely absent (Discomedusa, 

 Nausithoe). The mature generative products are dehisced into the 

 gastric cavity, and pass out through the mouth; but in many cases 

 the ova undergo their embryonic development either in the ovary 

 (Chrysaora) or in the oral tentacles (Aurelia). Separate sexes are 

 the rule. Male and female individuals, however, apart from the 

 colour of their generative organs, have only slight sexual differences, 

 as, for instance, the form and length of the tentacles (Aurelia), 

 Chrysaora is hermaphrodite. 



In the Discophora the development is generally accompanied by 

 an alternation of generations ; the asexual generations being repre- 

 sented by the Scyphistoma and Strubila; but in exceptional cases 

 it is direct (Pelagia). In all cases a complete segmentation leads 

 to the formation of a ciliated larva, the so-called planula, which 

 attaches itself by the pole which is directed forwards in swimming. 

 This pole is, however, opposite to the gastrula mouth, which in 

 the meantime becomes closed, while round the mouth, which is 



