Chap. V. THE GENUS POLYCLONIA. 143 



projecting towards the central cavity, without, however, communicating with it. In 

 Polyclonia, on the contrary, there are twelve main branches of the chymiferous 

 system, extending in a direct course towards the eyes, and forming a fork, between 

 the branches of which the eyes are placed (PI. XIII. Fig. 2, 7, o o). With these 

 branches alternate twelve somewhat smaller radiating chymiferous tubes, which are 

 lost in the network of anastomoses occupying the whole field between the main 

 branches. There is, however, a marked difference between these anastomoses. 

 Nearest to the margin, they are very small, and go on increasing towards the forks 

 of the main branches, between the base of which they are largest; while the space 

 nearer the main cavity is occupied by a netrwork of large meshes, formed, however, 

 by smaller lobes. The ramifications and anastomoses of the chymiferous tubes, along 

 the margin, are represented magnified in Fig. 9, a^, PI. XIIP. Eepeated injections 

 of this chymiferous system has satisfied me that the main radiating chymiferous 

 tubes, in the direction of the eyes, are afferent vessels, and that the stems, alter- 

 nating with them, which seem lost in the marginal anastomoses, are recurrent tubes, 

 through which the fluid passing from the main cavity, through the main branches 

 to the periphery, is Jjrought back to the main cavity. I am unable to say whether 

 there is a similar difference of function among the chymiferous tubes of Rhizostoma. 

 Occasionally the chymiferous tubes of Polyclonia present some irregularity in their 

 course, and the marked arrangement of the adult, just described, is not yet visible 

 in younger specimens (PI. XIII. Fig. 5), in which the anastomoses between the main 

 branches of the chymiferous system are comparatively few. 



The main cavity of the body is formed by the combination of the bases of 

 the eight arms arising from the thickened part of the lower floor, which closes 

 the lower side of that cavity. Fig. 4 of PI. XIII. shows these relations, t t, f f^, 

 f f, and f f rejiresenting the eight arms which form, respectively, the roimded 

 corners of the quadrangular cavity, sc sc marking the even thickness of the wall 

 above the origin of the arms, and oa oa the intervals between two and two arms, 

 corresponding to the sides of the main cavity, upon which open the holes leading 

 into the cavity below the genital pouches, oc oc. In this figure the main cavity 

 is seen from above, and its outer walls are cut immediately below the origin of 

 the radiating chymiferous tubes. The lower floor of that cavity is even, and from 

 it rise the walls of the four genital pouches, which project, like four lozenge- 

 shaped sacs, into the main cavity. In this figure, two of the pouches are removed, 

 so that the cavities, oc oc, which they cover, and which open outside, are visible ; 

 while the two other pouches (o os, as') appear in their natural position. The genital 

 organ proper {o o) forms a transverse band of folds across the middle of the pouches 

 which are kept in their respective position by the smooth fold of the pouch itself, 

 one part of which [os') is turned towards the centre of the cavity, while the other 



