278 CONTRIBUTIONS TO THE NATURAL HISTORY 



8, 9, e, fi,y", /), which, however, are much less powerful, and are placed half-way be- 

 tween the main bundles and the chymiferous tubes on both sides. These bundles follow 

 the saine course as the four others, and present the same relative disposition of their 

 fibres, but in such a reduced scale as not to be always apparent, and hardly ever to 

 appear as continuous vertical pillars of contractile fibres. Their existence, however, 

 cannot be doubted for a moment in certain states of contraction of the inner surface, 

 when the muscular vertical- bundles are shortened in such a manner as to form with 

 their pennate threads undulating lines curved in opposite directions, which, in a view 

 from above or below, intersect each other completely. Such a view is given in Plate III. 

 Fig. 7, 8, 9, where the main bundles, with their lateral fibrillae, are curved inwards, and 

 the eight secondary bundles outwards, meeting, in this case, similar curves formed by the 

 strong bundles. The greatest difficulty is experienced in forming correct ideas of all 

 these undulating lines intersecting each other, unless one has traced them in all possible 

 degrees of contraction. When the body is slightly contracted, and its circular form is 

 still preserved, as in Plate I. Fig. 3, 4, the intersecting lines are only those of the four 

 main bundles marked out upon the outlines of the gelatinous mass. But as the con- 

 traction increases, as seen in Plate III. Fig. 8, the undulating lines of the outer surface 

 are seen to correspond to the inner ones, and the eight secondary bundles coming into 

 play, instead of four arched lines there are now twelve, intersecting each other ; and in 

 Plate III. Fig. 7, the contraction being most powerful, each of these curves forms a 

 narrow arch, and all these arches intersect each other in the same way, to form now 

 an eight-rayed star instead of a more or less quadrangular figure, the chymiferous 

 tubes alternating with the four main muscles to form the curves bent inwards, and the 

 eight muscular secondary bundles arched outside to form the eight prominent angles of 

 the figure, the undulations of the external surface corresponding, even in that state, 

 to those of the inner surface. The correspondence of the external and the internal 

 vertical muscles is remarkably well shown in Plate HI. Fig. 7, 8, where a, the main 

 external vertical bundles, correspond to d, the main internal vertical bundles, and e and 

 f, the secondary internal vertical bundles, correspond to less regular secondary external 

 bundles, b, b. 



The third chief layer of contractile cells (Plate III. Fig. 3, 4, i, i) lines the main cavity 

 of the body, and presents a circular arrangement, so that, in its chief disposition, this system 

 crosses the vertical bundles everywhere at right angles. The existence of a continuous mus- 

 cular coat over the whole inner surface of the lower side of the disk would hardly be sus- 

 pected, unless upon the closest examination, and a careful selection of suitable specimens. 

 But whenever its constituent parts are in such a state of contraction as to render them 



