58 DISCOPHORiE. Part III. 



opening, as seen between the ai-ms (PI. VI. Fig. 1) and through the genital cavities 

 themselves (PI. VIT. Fig. 1), seem at first sight to be the natural outlets of the 

 sexual apparatus, and have generally been considered as such. Ehrenberg, in the 

 paper quoted above, has entirely overlooked the floors with double arches which 

 separate the genital pouches from the open cavities below, and has represented the 

 round opening of these cavities as leading directly into the genital sacs. See PI. 

 VII. Figs. 1 and 2, of his memoir. 



The natural consequence of this arrangement is, that the ovaries, which are 

 developed along the periphery of the lower floor of the genital sacs, discharge their 

 eggs into the cavity above that floor, from Avhich they have no other escape than 

 through the channels leading into the main cavity of the body, from which they 

 pass along the medial canals of the arms, into the little pouches formed by the 

 folding of their margin, where they undergo their first development. This structure 

 explains fully how it happens that, at the spawning season, the fringed margins of 

 the arms are so heavily laden with eggs (PI. VIII. Figs. 1 and 9). Were the eggs 

 discharged through the lower opening below the genital pouches, as Ehrenberg 

 supposed, they would immediately be scattered in the water, and could hardly be 

 gathered again into the folds of the arms; but following the course above described, 

 at the time when the arms have ceased to be very active, and when their mar- 

 gins ai"e brought into close contact with one another from both sides, it is hardly 

 possible that the eggs should readily escape ; and, indeed, we find that while they 

 accumulate in large numbers in the little pouches formed by the folds of the margin, 

 in which they remain even when the animals are shaken in the water, it is onl}^ 

 late in the season, when the margins of the arms begin themselves to decomjiose, 

 that the young, already in their planula state, are successively dropped. 



Having thus considered the general relations of these organs, we may now con- 

 sider more closely some other points of their structure. It is already known that 

 the Discophoraj have distinct sexes, but what is not so genei'ally understood is, that 

 at the spawning season, the males and females may readily be distinguished by 

 their external appearance. In our Aurelia, at least, the distinction is very easy. 

 In the first place, the oral ajjpendages of the females (PI. VI. Fig. 6) are much 

 stouter and thicker than those of the males {^Fig. 5), their upper side is more 

 ro\mded, while those of the males show a prominent keel, and the marginal fringes 

 are more extensively folded and the folds more intricately interwoven, preventing, 

 no doubt, the ready escape of the eggs in their undeveloped condition. It may 

 also be noticed, that even in their full-grown condition, the oral appendages of the 

 males are more pendant, while those of the females are usually coiled up. In 

 the second place, the ovaries are of a lighter, more yellowisli color, while the sper- 

 maries are more purplish, or rose color. At the time of spawning, this difference 



