Chap. II. STRUCTURE OF THE ADULT. 61 



iuterambulacral radiating tubes arise from the periphery of the sexual pouches, while 

 the ambulacral ones extend further inward between these pouches. The whole system 

 of these cavities, the radiating tubes, the sexual pouches, and the main digestive 

 cavity, are hollowed out, as it were, between the upper floor of the body, which 

 consists of the main gelatinous disk and is by far the thickest, and a lower floor 

 equally gelatinous, which is everywhere much thinner, as PI. VIII. Fig. 2 shows, 

 (magnified in part Fig. 3), though it is thickest below the genital pouches (PI. IX. 

 Figs. 7. 8, and 9 e) and at the base of the oral appendages {b), which are them- 

 selves a prolongation of that lower floor. A thorough comparison of the histological 

 peculiarities of our adult Aurelia, with those of the earlier periods of its growth, 

 remains a desideratum in its anatomical history. 



Professor H. J. Clark has furnished me with the following memorandum of an 

 investigation of part of this subject, upon which he has been engaged during the 

 last summer. "Excepting upon the doi'sal, or, as recently denominated by Pi'ofessor 

 Agassiz, the abactinal region of the disk, the outer and inner walls of the body 

 are underlaid by a thin, fibrous, muscular stratum. In that region of the actinal 

 side, which extends from the base of the tentacles to the outer margin of the 

 reproductive organs, the muscular fibres are fibrillate, and the fibrillte are arranged 

 in concentric circles, and are as distinctly striated as in the highest form of muscle ; 

 yet they are not arranged in fascicles, but lie side by side in a uniform succession, 

 from the inner to the exterior edge of the concentric series. In every other ^aart 

 of the body where the fibres are found, excepting in the marginal canal, they 

 trend radiatingly, and are neai'ly or altogether destitute of fibrillas and striae; these 

 features being detected with the greatest difficulty, and, after all, with some degree 

 of uncertainty. In the tentacles, and ocular peduncles, they run parallel to the 

 axis, and give them a longitudinally banded appearance ; in the marginal lobes, they 

 converge at their blunt apices ; and from the bases of the three above-mentioned 

 organs, they spread laterally, .and gradually pass into the concentric series. From 

 the inner edge of the latter, they again assume the radiating trend, and pass in 

 direct lines, without changing their course as they traverse the depths of the repro- 

 ductive pouches, to the base of the actinostome, and thence to the extreme border 

 of its four lobes. 



"Within the body the fibres trend radiatingly, traversing the cavity of the acti- 

 nostome in lines parallel to those Avithout, and then expanding in the digestive 

 cavity, they pass directly to its border, following all the convolutions of the rejiro- 

 ductive organs, and then entering the radiating canals, they course longitudinall}- 

 to the point of junction with the marginal canal, where they diverge laterally, and 

 follow a circular direction along the channel and parallel to the margin of the 

 disk. At the bases of the tentacles, the interior and exterior muscular layers unite 



