﻿OF THE ACALEPHJE OF NORTH AMERICA. 267 



from below, whenever a perfect view of the whole circular tube is obtained, and is 

 actually drawn as a thread with double outlines in the figures above quoted, as well as 

 in those representing the muscles, and more particularly in Plate II. Fig. 11, a, 14, f, 16, c, 

 and 17, b, b. Under a low magnifying power, this cord appears simply as a thread following 

 the chymiferous tube. (Plate I. Fig. 2, 3, 4.) When I first noticed it, it appeared to me as 

 a small thread ; and though its structure was not fully recognized then, its connection with 

 the sensitive bulbs led at once to the appreciation of its real nature as the main cord of 

 the nervous or sensitive system. The difficulty of distinguishing between the tissues in 

 this part of the body where so heterogeneous organs meet together, has rendered it for a 

 long time difficult for me to ascertain the real nature of this cord. For here we have 

 also circular muscles, or contractile fibres, which move the transverse partition of the 

 lower part of the disk ; we have an outer and an inner epithelium ; and finally, the walls of 

 the chymiferous tubes, which, at times, present themselves with so strongly defined out- 

 lines, that when the tube is not in its natural position, but more or less twisted, the sen- 

 sitive cord does not appear on its margin as a distinct thread with peculiar outlines. But all 

 the conditions for correct investigation of the facts having been assumed, I have repeatedly 

 satisfied myself and others, that there is, along the inner margin of the chymiferous tube, 

 a cord differing in its microscopic structure from all the contractile fibres and from all 

 the epithelial cells around it, which follows the whole course of this circular tube, and 

 enters into the lateral corners of the sensitive bulbs, where it forms part of their mass. 



This cord, examined under high magnifying powers, such as Ocular 3, Objective 8, of 

 Oberh'auser's microscope, appears as a string of several rows of nucleated cells, ovate in 

 their form and placed with their longer diameter in irregularly continuous lines (Plate III. 

 Fig. 13) ; that is to say, the cells are not strictly placed in juxtaposition by their ends, 

 but alternate more or less, so as to form a cord-like mass, the elements of which are ovate 

 cells placed side by side, their tips interposed between each other. This arrangement and 

 the form of the cells make it easy, after the organ has once been seen, to recognize it 

 again in whatever position it may be observed, and of late I have frequently been able 

 to trace it upon the circular tube, passing obliquely across it, or following it in an oblique 

 direction, whenever the parts had been twisted. Tracing this cord for the whole course 

 of the circular tube, we see it penetrate into the angles of the sensitive bulbs, (Plate II. 

 Fig. 11, a, a, 14,/,/, and 16, c,) in which its cells mingle with colored pigment-cells in 

 such a manner as to form a compact ganglion of heterogeneous elements, rather than a 

 nervous bulb. Of this, however, presently. Let me first remark, that in Plate III. 

 Fig. 14, the nervous cord, c, is figured as it is usually noticed between the other organs ; 

 a being the sensitive bulb, b the chymiferous tube, c the sensitive cord, and d the 



