144 DISCOPHOR^. Part III. 



part (os) is turned toward the openings of the wall of the cavit}^ which com- 

 municate with the surrounding medium. From this arrangement, it is evident that 

 the genital pouches cannot be turned inside out through these openings, as is the 

 case in the Cyaneidge, though water is constantly floAving in and out, in conse- 

 quence of the expansion and contraction of the pouches themselves. Along the 

 edges of the sexual organs there are short, hollow tentacles, projecting inward, 

 which, by their motion, must contribute to the aeration of the eggs, by the con- 

 stant change of the surface of the water with which they are brought into contact. 

 These tentacles are homologous to the digitate appendages of the sexual organs 

 of Aurelia. 



Between the four genital pouches there are four openings in the lower floor 

 (s s), magnified in Fi</s. 6 and 7, PI. XIII., which lead into the main channels ti'av- 

 ersing the arms, and communicate, therefore, with the surrounding medium, through 

 the narrow apertures or pores scattered between the fringes of the arms. Through 

 these pores the food is introduced into the branching channels of the arms, and 

 through these into the main cavity, into which the apertures (s), above described, 

 directly lead. As the mature eggs fall into the main cavity, they have no other 

 way to make their escape except through these same apertures and channels. As 

 these apertures, s s, Fig. 4, are the only openings through which the food reaches 

 the main cavity of the body, they might be considered as mouths, but it would 

 certainly be a violation of all homologies, to call by this name openings which are 

 removed from the holes leading to this cavity by the whole distance of the length 

 of those parts of the arms where they communicate with the surrounding medium. 

 Far, therefore, from being mouths, they are truly homologous with those emarginations 

 in the angle of the arms, in Aurelia (see PI. VI. Fig. 3, i /), which also lead into 

 the main cavity of the body, and we must look for the mouth elsewhere. Now, 

 a comparison of the arms, represented PL XIII. Fig. 2, segments 4 and 5, with the 

 arms of Aurelia, represented PL VI. Fig. 1 (where their marginal lobes are closed 

 upon one another), and Fig. 3 (in which the same marginal lobes are spread open, 

 to show how the capillary surface inclosed between these mai'gins lead into the main 

 cavity of the body), will leave no doubt upon the mind of an unprejudiced observer, 

 that there is no essential difference between the structure of the arms of Poly- 

 clonia and Aurelia, except in the mode of branching of the whole arm, and the 

 closer approximation of their margins in Polyclonia, in which they are soldered at 

 intervals, and cannot, therefore, be spread, as those of Aurelia. Figs. 15 and 16 

 show these margins, and the way in Avhich their terminal lobules are apj^roximated, 

 leaving, here and there, wider fissures between them. In tact, but for the con- 

 nection between opposite margins of the same arm, the structure of these parts 

 is the same in Aurelia and Polyclonia. Fig. 7 of PL VII., which represents a por- 



