i;!t DISCOPllOR.E. Paut 111. 



(lini'rent genera, and vary Iruni lour to t'iglit, as do also the openings facing the 

 genital sacs. Secondly, the stem, or central disk, towards which these roots con- 

 verge above, and in the centre of which there is a cavity in the form of a cross. 

 Prom this point the walls of that cavity branch again, radiatingly, and form, thirdly, 

 the so-called arms. The arms themselves may be uniform throughont, and exhibit 

 only a swelling near their extremity, as in Tje[)tobrachia ; or there may be a bimch 

 of ramifications near the base, and the remainder of the arm be a, simple thread, 

 as in Cephea; or there may be two bunches of ramifications, at a distance from 

 one another, and a simple termination to each arm, as in Rhizostoma; or the whole 

 arm may be uniformly branching as in Polyclonia. 



Another point of importance is the degree of independence or isolation which 

 the central disk, intervening Ijetween the pendant arms, acquires from the arches 

 or roots of the arms, from which it is derived morphologically, and the character 

 and com]ilication of that disk. In the Cassiopcaj the central disk seems raised, 

 as if detached from the surrounding parts of the lower floor, and completely inde- 

 pendent from the side walls of the main cavity. So it is, also, in Leptobrachia 

 and in Cotylorhiza ; but in lihizostoma it is confluent with the basal arches of 

 the arms, which alternate with the genital sacs, so that this part of tlie actinostome 

 differs least in Rhizostoma from the ordinary structure it exhibits in the Aurclidte 

 and Cyaneida*. 



The relations of the arms to the eyes or marginal ocelli are equally important. 

 In Rhizostoma, which have four genital sacs and four oral arches, there is one 

 eye in the radius of each ovary, and one in the radius of each oral arch. In 

 the CassiopCtT?, Avhich have eight genital sacs and eight arms, apparently indepen- 

 dent of the oral arches, there is one eye in the radius of each ovar}^, and the arms 

 alternate with the eyes. The relations of the cross of the mouth are not easily 

 defined ; it seems, however, to correspond to lour of the iinns, and not to four 

 eyes. In Polyclonia, the four arms are likewise in tlie radial prolongation of four 

 eyes, but there iwv no eyes fronting the radial prolongation of the centre of the 

 lour genital sacs, though there is one eye in each segment of the disk Avhich 

 alternates with the oral segments and the centre of the genital segments. Elabo- 

 rate as the figures of Cotylorhiza, published by Delle Chiaje, seem to be, they do not 

 represent the marginal ocelli. In Leptobrachia, which has four genital sacs and 

 eight arms, which are also independent of the oral arches, it would seem, from the 

 figures of Chamisso, that there are four arms alternating with four ovaries, and 

 corresponiling to the angles of the cross of tiie mouth, and four facing the ovaries 

 and alternating with the cross of the mouth. But such a combination is so con- 

 trary to the symmetry of the Acalephs, that I suspect here an error of obser- 

 vation. The position of the eyes cannot be ascertained from the figures thus far 



