﻿280 CONTRIBUTIONS TO THE NATURAL HISTORY 



fibres contracting more powerfully than the upper ones, that I hardly entertain a doubt 

 with respect to their cooperation in the locomotive process. But, had not this system a 

 particular function, I hardly see why it should be so well developed in the upper part of 

 the inner cavity, which is hardly liable to great changes arising from muscular action. 

 The fact that these horizontal or circular fibres line the arched cavity gives to the fibres 

 themselves, when seen in different positions, such various aspects, that here again it is 

 very difficult to form correct views of the whole system ; especially so, as very often the 

 curves of the horizontal fibres coincide with the curves of the fibres diverging from the 

 pennate vertical muscular bundles, in such a manner as to cover each other in the per- 

 spective, or to be easily mistaken one for the other. And, in such views, the horizontal 

 circular fibres contribute to strengthen the appearance of the intersecting undulating 

 lines formed by the vertical bundles already described, especially when seen from 

 above ( Plate III. Fig. 9, i). But in profile views, the fibres belonging to the cir- 

 cular system are readily distinguished from the fibres belonging to the vertical bundles, 

 even when they assume an arched appearance, by the circumstance that the former are 

 attached to the chymiferous tubes, as seen in Fig. 3 and 4, while the latter alternate 

 with these tubes. About the chymiferous tubes, there are, however, more or less distinct 

 threads, especially upon the inner surface, which seem to intersect the horizontal circular 

 system, and nevertheless neither to belong to it nor to the vertical muscles. They rise 

 (Fig. 3, 4, g, g) chiefly from the middle of the height of the main cavity, and diverge 

 towards the main vertical muscles, following, however, the form of the main cavity, upon 

 the inner surface of which they are appressed, and intersecting under various angles both 

 transverse and vertical fibres. But for these fibres, I should not have the slightest doubt 

 respecting the real nature of those other fibres which I have ascribed to the nervous sys- 

 tem ; and had I noticed a plexus-like crossing of these fibres with each other, I should 

 have given up the idea that the meshes of fibres under the chymiferous arches are sensi- 

 tive plexuses. But here the oblique fibres intersect fibres of other systems, and do not in- 

 tersect each other, as in the plexuses described above. 



In addition to the difficulties depending upon the nature of the subject itself, which 

 we experience in tracing these muscular bundles, there is another difficulty, arising from 

 the position in which we trace them. There being four radiating tubes, the whole animal 

 assumes a very different appearance, according to the position in which it is viewed. 

 When we look at it in such a position that two tubes are placed upon one side and two 

 upon the other, as in Plates I. and III. Fig. 1, we face the flat side of a broad surface 

 stretching between the two tubes nearest to us in the profile. But if we place the ani- 

 mal in such a position as to have right and left a single tube, and one upon the middle 



