(4; 



EXPLANATION OF THE PLATES. 



leaves only tubes for tbe perii)horie cin-ukUiou of the 

 fluid contained in it. Tlie vertical prolongation/ of this 

 main cavity extends in the direction of the circumscribed 

 area, and branches into two forks y, /^, at its termi- 

 nation. The other tubes arising from it are the two main 

 chyuiiferous horizontal tubes e, e, with their branches q, 

 (/, and their eight ambulacral tubes ? to P, which open 

 into the vertical tubes ? to P. The tubes )•, ?•', which 

 follow the walls of the digestive canity, arise also from 

 it near the main horizontal trunks ; and from these latter 

 arise the tubes of the tentacular ajjparatus a, a. 



e, e, the main horizontal trunks of the chymiferous tubes, 

 from which arise the eight radiating branches opening 

 into the ambulacral tubes. 



/, the vertical or axial funnel-like prolongation of the main 

 cavity of the body. /', /"■- the two forks of that fun- 

 nel. It should be remarked, that the direction of that 

 fork is in the plane of the longest diameter of the cir- 

 cumscribed area, which is also the direction of the 

 longitudinal diameter of the mouth. 



g, the roots of the tentacle ; (/' the edge of the ridge 

 of the tentacular base ; g" the side of the ridge. 



/(, 7i', /(-". — h designates the whole tentacular apparatus 

 with all its complicated parts, A' being the tentacidai- 

 apparatus of one side, and Ir the tentacular apparatus 

 of the other side. These uumbei'S are appropriated to 

 the same appai'atus in every iigure, whatever may be 

 the position in which the animal is observed. It wiU 

 be noticed, that these tentacles are placed at right 

 angles with the plane of the mouth and of the circum- 

 scribed area. 



i, the eight horizontal tubes of the chymiterous apparatus 

 which reach the vertical tubes, following the vertical rows 

 of locomotive flappers. In all the figures the horizon- 

 tal tubes are numbered in the same way, beginning 

 with Xo. 1 and ending with Xo. 8. Xo. 1 is assigned 

 to that tube which extends to the vi'rtical row in 

 sight on the U'ft hand when the mouth is turned ujj- 

 wartl and the tentacular apparatus appears symmetri- 

 cally on the right and on the letl ; so that P, r, P, 

 i' are the four horizontal tubes of one half of the 

 body, and i', p, (', i* are the four horizontal tubes of 

 the opposite half. And if the view I have taken of 

 the diameters of these animals is correct, that the longi- 

 tudinal diameter of the mouth divides the body into 

 synunetrieal halves, one to the right and the other to 

 the let\, the tubes i^ to i* are the tubes of the an- 

 terior half, and the tubes ? to P are the tubes of the 

 posterior half, and the tubes P, P, P, P are the tubes 

 of the left side, and the tubes i'', i\ P, P are those of 



the right side, or ctcc versa, as we can only establish 

 these general relations between the difliY'rent diameters 

 without determining strictly which is the anterior and 

 which is the posterior edge of the mouth. It is prob- 

 able, however, that no distinction is intended in the 

 structure of these animals, as they are capable of as- 

 suming inverse positions, mouth upward and mouth 

 downward, in which case the edges of the mouth 

 appear in an invei-se position. 



j, the tentacular socket or cavity in which the tentacular 

 apparatus is suspended, and to the inner wall of which 

 it is attached. This cavity opens at _/', and through 

 this opening the tentacle may be extended ; but it is 

 also capable of such contraction as to be entirely with- 

 drawn within the cavity /. 



/', opening of the tentacular eavit}-, through which the 

 tentacle is protruded. 



Ic, the main stem of the tentacle from which the fringes 

 arise. 



k', fringes of the tentacles which arise uniformly upon 

 the same side, the outside, of the tentacle, so that 

 they are stretched in opposite directions from the two 

 sides. But this direction is constantly modified in 

 the various attitudes and the various degrees of elon- 

 gation of the tentacles, as these are capable of being 

 twisted upon themselves ; so that the fringes may 

 appear as forming a spiral upon the main stem, or 

 may be stretched in all possible directions, in their 

 more or less extensive elongations. However, at the 

 base they arise strictly in opposite directions. 



/, /, the vertical rows of locomotive flappers, of which there 

 are eight of uniform length in Pleurobrachia. These 

 vertical rows are numbered in the same manner as the 

 horizontal tubes which open into the vertical chymiferous 

 tubes accompanying the flappers, and these uumbei-s 

 con-espond in the diflerent figures, in the same man- 

 ner as ill the tubes ; P to /* being the rows of one 

 extrcmit}-, and P to /' those of the other extremity, 

 and P, P, P, P being the rows of one side, and P, P, 

 P, P the rows of the other side. 



»(, the radial ccUulo-motor system around the corners of 

 the mouth. 



Hi', the oral motor S}stem. 



m-, the radial system in the tentacular plane. 



iiP, the lateral system where it passes from the actinal 

 en<l of the tentacular sockets to the perijihery of the 

 body. 



n, the interambulacral motor bauds iu the plaue of the 

 digestive cavity. 



ip, the same as n, but in the teutacidar plane. 



