ACALEPHAE. 159 



respond to the mesenteric filament of the Anthozoa, and lend the 

 same aid to digestion by the secretion of their glandular endodermal 

 covering. They are always attached to the sub-umbrella wall of the 

 stomach, and fall in the four interradii, i.e., the radii of the generative 

 organs which alternate with the radii of the angles of the mouth 

 (radii of the first order). They usually follow the inner edge of the 

 generative frill in a simple or curved line. 



The nervous system and rhopalia have already been described 

 (p. 115). 



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

 tinguished in consequence of their size and their bright colouring. 

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

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

 pits (hence the term Phanero'-arpae Esch.). In all cases these bands 

 lie on the lower (sub-umbrella) wall of the digestive cavity, from 

 which they originate as leaf-like prominences. Their upper surface 

 is covered with gastric epithelium ; their 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 subgenital pits have already been 

 described (p. 118): they may be completely absent (Ephyra, 

 Xausithoe) : their lining consists of sub-umbrella ectoderm and is 

 quite distinct from the generative epithelium, which is of endodermal 

 origin. 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 

 (Chrysa&ra), or in the oral tentacles (AureUct). 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 (AureUa). 

 Chrysaora is hermaphrodite. 



In the Epliyroninae the development is generally accompanied by 

 an alternation of generations; the asexual generations being repre- 

 sented by the Scypliistoma (Hydra tuba) and Strolnla ; but in 

 exceptional cases it is direct (Pelayici). In all cases a complete 

 segmentation leads to the formation of a ciliated larva (Fig. 132 a), 

 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 formed as a perforation at the free end, 

 the tentacles appear (Fig. 132 b, c). As in the embryo Actinia, two 



