OF THE ACALEPH* OF NORTH AMERICA. 297 



flat upon the bottom of the glass plate, upon which it is brought under the microscope. 

 It is therefore of the utmost importance to be first thorouglily acquainted with these 

 changes of form of the whole animal, and especially with the unnatural fold which is 

 formed at some distance from the margin in the contraction of death, before any further 

 attempt is made to investigate the connections of the muscular apparatus of either the 

 surface or the inner cavity of the disk. As soon, however, as we are familiarized 

 with this difficulty, it is easily ascertained that there are in the disk three muscular 

 systems, as in the preceding genera. For, besides the superficial system already men- 

 tioned, we trace, with the same ease, another on the inner surface of the gelatinous 

 disk, which has the same general arrangement, its main bundles alternating with the 

 radiating chymiferous tubes, and smaller bundles arising between these and the tubes ; 

 so that there are four chief bundles extending from the summit towards the lower mar- 

 gin of the inner surface of the gelatinous mass, and eight smaller ones alternating be- 

 tween these and the radiating tubes, besides some fibres which follow the tubes and 

 have a more pennate arrangement. The only difficulty there is in tracing these mus- 

 cles is in ascertaining that the superficial bundles and the inner bundles constitute two 

 distinct systems ; for here the gelatinous disk is so much thinner than in either Sar- 

 sia or Hippocrene, as to leave it frequently doubtful whether we have the fibres of the 

 upper, or those of the lower surface in the focus of the microscope. Again, owing to 

 the unrolling of the margin where the fold of the peripheric part of the disk is formed 

 upon the more massy portion of the centre, there seems always to be a complete in- 

 terruption in the course of the muscles ; so that it requires more practice with these 

 animals than with other genera before their contractile fibres can be traced for their 

 whole length, the radiating chymiferous tubes themselves seeming frequently interrupted 

 at the same place. I mention this difficulty for the sole purpose of removing at once the 

 doubts which might arise in the minds of naturalists induced to repeat these observations, 

 who otherwise might not at first perceive the cause of a discrepancy between the illustra- 

 tions given here, and the appearance of specimens as they are seen at times under the 

 microscope, though these circumstances have no reference to the real structure of the an- 

 imal itself. 



The third muscular system of the disk lines, with concentric circular fibres, the inner 

 cavity, or rather the lower surface of the gelatinous disk. This system seems to be very 

 powerful, even more so, comparatively, than in other genera ; and, if we are allowed to 

 trust the appearances, there seem to be, at a slight distance from the tentacular margin, 

 about the lower extremity of the ovaries, stronger bundles of these fibres, which, when 

 fully contracted, break the regular continuity of the outlines of the surface, forming a sort 



