MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 91 



hydrocu'lc. Radial iwiieli 1, linally, after liaviuy passed around the (esoi)liagiis, comes to rest 

 at the point where it originated. Radial jjouch o, it will be noted, i.s carried only half as far 

 as radial i>oucli 1, or from its jioint of orij-in on the left to a i)oint o])i)osite on the right of the 

 a'SOi)hagus. (Compare ligs. (i and !l, (1) and (.")).) 



Tbi.s great amount of rotation seemed so jieculiar that I hesitated for some time to believe it, 

 and was led to suppose instead that while the hydroco'K- moved to the right the other organs 

 lying above it rotated an ecjual amount to the left. 



The early closure of the blastopore and the central position of the mouth in the early stages 

 make such a view as tlie latter seem possible, and as it may suggest itself to those who study figs, 

 fi and V, 1 will give below the i)oints which seem to me, directly or indireirtly, to i)rove that the 

 bydrociele revolves under the euterocceles and stomach, rather than that the latter twist over 

 the hydroco'le: 



(«) The ectodermal bulges, nearer the posterior end in •• C" (tig. (i), are the same as those 

 nearer the posterior end of " I)" (lig. !l), on which the third ciliated band is situated. 



(b) If tlie latter view is the correct one then radial canal 3 points toward the same end of 

 the larva in both "C and "U" (tigs. G and !•), but in "C" the end toward which it ])oints is 

 anteriorly directed in swimming and in " 1)'' it ])oiMts away from the end which ])recedes. It is 

 hardly thinkable that in any stage in its development the anterior end of a larva should change 

 its physiological function and become the ]iostcrior end. 



((■) By any other view than the one I have adopted the blastopore, or the i>oint where it 

 existed before closing, would W anterior and the larval organ posterior in i)osition. In all known 

 echinoderm larvpe., however, the blastopore marks the posterior end, and in all cases where it 

 occurs the larval organ originates from the anterior end of tlie larva-. 



(d) It may be recalled, also, that in tiie readjustment of jiarts which takes place during the 

 metamorpliosis of other echinoderm larva' the rotation is almost entirely contined to the hydroc<i'le. 



As the hydrocd'le ])asses around the (esophagus the tube counecling it with the left horn of 

 the hypogastric enterocode becomes broken and the left anterior eiiterociele, together with the 

 tube connecting it with the left horn of the hydrocode, are carried anteriorly around the o'sojihagus 

 (fig. !), ael and st). In "D," then, we find tiie stone canal on the right side of a line dividing the 

 larva into symmetrical halves, instead of to the left of the same line as it is in "C." (Compare 

 figs. (> and 7 with 0.) The anterior enterocode comes to rest immediately in front of the stomach 

 and (esophagus. 



From the point where the stone canal enters the anterior enteroi-ulo the pore canal grows 

 out, passes dor.sally to the ectoderm, with which latter its walls fuse, and an oiieiiing the water 

 pore (tigs.!) and 11, pc) breaks through. Thus in tliis stage the codom and liydrocode are first 

 connected with the exterior. 



In '-C the circular water-canal liad not closed, but existed in the form of a horseshoe, the 

 concave side of which opened posteriorly, but as the rotation of the liydroc<ide takes place its 

 horns grow toward (Mch other until they meet. A fusion of their walls then takes place at the 

 point of contact and a complete ring is thus formed. The part of the ring canal, the Ibrmation of 

 which has just been described, lies between radial canals 1 and ."■> in fig. !•. The opening of the 

 stone canal into the water ring is situated in "C" at the base of radial canal 5, but by means of 

 the rotation of the hydrocele about the (esophagus, togettier with the growth of the ends of the 

 horseshoe, this ojieniug is carried away from its position at the base of radial canal 5 toward 

 radial canal 1. It always remains, however, nearer the former than the latter; in other words, it 

 comes to lie definitively in the right iKinidiii.i between radial canals ."> and 1. (Compare tigs. 6 

 and !).) 



The radial canals, which existed in "C" as simi)Ie iiouches from the convex side of thehydro- 

 cade, have in " D" each become threelobed. Near the tip and froiii the sides of eadi canal a jiair 

 of pouches has budded out, each of which is about e(iual in size to the end of the canal which lies 

 between and beyond them (fig. 0, et and tl). In these three structures we have the rudiments of 

 th(^ end teiita<de and the first jiair of foot tentacles of the ophiurid arm. 



When we were last considering the hypogastric enterocode it was in the form of a crescent, 

 the horns of which were very short and its central part very wide. Into its concavity, which was 



