OF THE ACALEPH>« OF NORTH AMERICA. 281 



lino ill front covering another behind, we at once place the plane between the tubes in 

 an oblique position, and every thing in a different light. And these two positions, in 

 which it is, after all, easiest to study the parts of the animal, may be still further compli- 

 cated by introducing interme^diate positions ; such, for instance, as that represented in 

 Plate III. Fig. 5; and the difficulties are beyond tlie patience of the observer, if, in addi- 

 tion to these numerous front positions, the difficulties of oblique positions are introduced, 

 in which partial views from above are combined with partial profiles. I have spent weeks 

 in attempting to make myself familiar with these different views, and if I were not con- 

 scious of having overcome most of the difficulties connected with the investigation, I 

 should by no means be so positive about the direction of the fibres of the muscular sys- 

 tem in the different layers described above. 



The region about the upper part of the digestive cavity presents such a complication 

 of different parts brought in connection with each other, that it is very difficult to form 

 correct ideas of them, and still more to appreciate their forms and connections, when 

 seen either from above or in profile. There is, especially, one point which puzzled me for 

 a long time, — the fact that, in the upper part of the arches of the vertical tubes, all the 

 outlines seem double. This is partly owing to the circumstance that the mere form of 

 the main cavity of the body gives rise, under different intersections of light, to the for- 

 mation of outlines which have no real existence, and that these outlines, seen across the 

 gelatinous mass from the two sides, are reflected upon each other. But after this difficulty 

 has been overcome, and we see the structures in their true simple character, there is still 

 a difficult point in defining the outlines of the upper transverse muscular bundles stretch- 

 ing between the arches of the chymiferous tubes, and in distinguishing them from the 

 circular nervous thread, which connects the nervous plexuses under the arches of the chym- 

 iferous tubes ; for here, again, the muscular fibres run so nearly in the same direction as 

 the nervous thread, that, but for the contractility of the one and the steadiness of the 

 other, they would be easily mistaken ; and I am not fully sure that, even after alluding to 

 this difficulty, they will always be correctly seen, especiall}' when examined from above. 

 A comparison of Fig. 4 and 5 of Plate III., in which the nervous system (Fig. 5) is 

 represented in a position of the animal almost identical with that in which we see the 

 system of circular muscular fibres. Fig. 4, will at once give an idea of the amount of diffi- 

 culty there must be in identifying these various parts, especially when it is remembered, 

 that both the nervous and muscular systems have to be studied through the thickness of 

 the gelatinous disk. Seen in absolute profile, the difference is more obvious, for here 

 the muscular bundles are so arched as to contrast with the nervous fibres stretching 

 across the same space. 



