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RITTER 



able structure. Its resemblance to the branching ' tap-root ' of some 

 plants Is striking enough. The peduncle proper is usually as long as 

 the body of the animal or longer. It is always thread-like, or lash- 

 like. At its junction with the body it is considerably larger in diam- 

 eter than it is elsewhere, but it diminishes abruptly to the small size 

 which it holds for most of its length. 



The root-hairs are not scattered vmiformly throughout the length of 

 the peduncle, but are in the main grouped in tufts with a few single 

 ones given off here and there. Sand grains cling tenaciously to the 

 hair tufts and increase the resemblance to a plant root (p1. xxviii, figs. 

 lO and 13). 



The minute structure of the peduncle and rootlets is peculiar, and 

 quite unexpected in view of the structure of similar growths in other 

 Ascidians. The smallest of the root-hairs, examined with a high power, 

 have a striking resemblance to a medullated nerve fiber. Each con- 

 tains a central fiber, homogeneous in appearance and of regular out- 

 line that would correspond to the axis cylinder ; and a transparent, 

 structureless outer portion forming the greater bulk of the whole, that 

 would correspond to the medullary substance (p1. xxviii, fig. 15). 

 Each of these rootlets, if traced distally, is found to terminate in a more 

 or less bulbous enlargement, the main structural element of which is 

 the thickened and closely tortuous end of the central fiber. The outer 

 transparent substance of the bulb does not differ essentially, either in 

 structure or relative amount, from that of the intermediate portions of 

 the rootlet. In some instances the bulbs are merely slight enlarge- 

 ments of the rootlet, the length being considerably greater than the 

 thickness. In others again they are thicker than long and are set off 

 from the rootlet by a pronounced shoulder. In these latter cases the 

 central fiber is often converted into a considerable mass by being itself 

 somewhat enlarged, and by being thrown into close windings. 



By tracing the rootlets, proximally to their origins in other secon- 

 dary roots (p1. xxviii, fig. 15) or in the main stem (p1. xxviii, fig. 14) 

 one finds that the central fibers enter the main stem, either directly or 

 by continuing to it through a larger rootlet, and then run on, preserv- 

 ing their individuality, up through the stem to the body of the animal 

 (p1. xxviii, fig. 14). Thus it follows that the peduncle, at its junction 

 with the body, contains a very large number of these fibers — as many 

 as the entire number of rootlets. 



The nature of the axial fibers is not obvious at first sight. Atten- 

 tive examination, however, proves them to be muscle fibers. The 

 chief difficulty in the way of immediately recognizing their true nature 



