282 CONTRIBUTIONS TO THE NATURAL HISTORY 



There is another point which requires particular attention. Owing to the thick- 

 ness of the chymiferous tubes, the transverse muscular coat of the inner surface is some- 

 what separated from the inner surface of the gelatinous disk near the tubes them- 

 selves, and these spaces left between the disk and the layer of muscles on the sides 

 of the chymiferous tubes give rise to a double contour in the uppermost part of the arch, 

 which, in addition to the difficulty already mentioned, is likely to create misapprehension 

 in estimating the appearances presented by the parts meeting about the centre of the disk 

 under the gelatinous mass.* Indeed, seen from above, it would seem as if there were a 

 distinct circle of peculiar organs, or rather of peculiar tissue, the periphery of which 

 would inclose the space to which the prominent corners of the digestive cavity reach, as 

 seen in Plate III. Fig. 7 and 9, and also Plate I. Fig. 3. The fallacy of this outline is fur- 

 ther increased by the appearance of the four main bundles of vertical muscles, which arise, 

 as it were, from those of the circle, (Plate III. Fig. 9,) and stretch downwards. The 

 outline of these circular fibres is sometimes made very prominent when the opening of 

 the lower partition is reduced by contraction, as in Fig. 7. Both circles, however, are more 

 distinct and farther apart when the outline of the lower partition is in a state of relaxa- 

 tion, as in Fig. 9. This outer circle, as seen from above, is a real outline. It is the free 

 margin of the lower partition, k, which is as definite as the outline of the lower circular 

 tube, 0, or the nervous cord, m, which in such views are all brought together in focus ; 

 but the middle upper circle has no more reality than the undulating lines d, e, f, seen out- 

 side of the chymiferous tubes, in Fig. 7, 8, 9, and 4, — which are only the foreshortened 

 outlines of the undulations produced by the contraction of the muscular system, — with this 

 difference, that the upper circle is formed both by the muscular coating, and by the influ- 

 ence which the prominent gelatinous knob in the centre has in causing an outline to ap- 

 pear round its base, where it bends over into the arch of the sides. But the double out- 

 line between the muscular coating and the arches of the chymiferous tube is hardly ever 

 noticed from above ; neither is it noticed in the complete profile ; but it is constantly and 

 distinctly seen in a three-quarter view (Plate III. Fig. 2, and also Fig. 4) ; and I must 

 confess that I never met with more perplexing difficulties than those I experienced in 

 satisfying myself of the real nature of what so clearly seemed a structure, which always 

 vanished under certain influences of light, while it was so plain under others. 



The fourth muscular system is extended between the four sensitive bulbs within the 

 lower margin. Plate I. Fig. 2, 3, and also Plate III. Fig. 9, k, and Fig. 2, /, /, give 

 an excellent view of it, in its general outline, though no structure could be drawn with 



* Upon this particular point, compare also another remark already introduced above, when describing the cir- 

 cular fibres. 



