HYDROZOA ACRASPEDA. 785 



wall of the gastric cavity centrally to the genital organs. Four single 

 filaments only are present in the genus Ephyra. The endoderm covering 

 them consists of ciliated cells, gland cells, muscle cells, and cnidoblasts. 

 The genital organs are either ridge-like or lamellate, and project into the 

 gastric cavity. They are situated on the inner aspect of the thin gastro- 

 genital membranes (p. 783) ; and the genital products are derived from the 

 endodermal cells of the peripheral or lower face of the ridges or lamellae. 

 The original shape of the ridge is that of a horseshoe, with the concavity 

 turned to the margin of the bell, but in the majority of Semostomae and 

 Rhizostomae the curvature is reversed. The ridge is rarely divided into 

 two parts, as in a few Cannostomae, but it is very generally converted into 

 a lamella which may become lobed 1 . The gastrogenital membrane may 

 increase in size, become folded and hang down into the cavity of the bell in 

 some Semostomae^ e. g. Pelagia, Cyanea ; or be invaginated into the gastric 

 cavity, e. g. in Aurelia, most Rhizostomae. The walls of the subumbrella 

 surrounding the four membranes may be much thickened, and consequently 

 the membranes come to lie at the bottom of subgenital cavities or lemnia, 

 as in Aureliadae and most Rhizostomae. The aperture into the cavities 

 may, in the last-named, be greatly reduced, or covered by an axial and 

 abaxial process of the subumbrellar wall, so that each of them acquires 

 the appearance of a double cavity. In the Rhizostome Versuridae and 

 Crambessidae the four gastrogenital membranes meet in the centre of the 

 bell. The basal portion of the manubrium is consequently resolved into four 

 hollow pillars which correspond to its four angles. The cavity thus formed 

 is the ' sub-genital portico ' or syndemnium 2 . The Semostome Chrysaora 

 is hermaphrodite. Sperm sacs occur upon its genital lamellae, which pro- 

 duce ova in a normal manner, as well as upon the subumbrellar wall of the 

 gastric cavity, in the gastric pouches, and on the arms 3 . 



Variations in the number of the radial segments sometimes occur. 

 Aurelia aurita is particularly liable to such malformations, and in it the 

 number may be increased from eight to twelve, or diminished to three or 

 four. The bell may be affected alone or the manubrium, and the genitalia 



1 The primitive ridge is converted into a lamella by the growth of endoderm cells from its 

 margin into the mesoglaea of the gastrogenital membrane. Connecting pillars of cells, or of 

 mesoglaea covered by cells, are often left between the genital lamella and the gastrogenital 

 membrane. See von Lendenfeld, Z. W. Z. xxxvii. p. 536 et seqq. ; and Glaus, Untersuchungen, p. 

 33 et seqq. and p. 38. 



2 Von Lendenfeld appears to have observed the formation of this chamber in Stylorhiza 

 punctata. He states that it occurs as Haeckel supposed, i. e. by the encroachment of the gastro- 

 genital membranes on the base of the manubrium until they meet and fuse. See Proc. Lin. Soc. New 

 South Wales, ix. p. 307, or Z. A. vii. p. 431. In the first-named paper the author describes the 

 Medusa in question under the generic name of Phyllorhiza. This is an error, because Phyllorhiza is 

 a Pilemid genus, and there is no subgenital portico in Pilemidae. 



3 The subumbrellar saccules of certain Linergidae (Cannostomae} are perhaps testes. See 

 Haeckel, System, p. 493. 



3E 



