MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 91 



hydrocoele. Radial pouch 1, finally, after having passed around the (esophagus, comes to rest 

 at the point where it originated. Radial pouch 5, it will be noted, is carried only half as far 

 as radial pouch 1, or from its point of origin on the left to a point opposite on the right of the 

 (esophagus. (Compare figs. G and 9, (1) and (5).) 



This great amount of rotation seemed so peculiar that I hesitated for some time to believe it, 

 and was led to suppose instead that while the hydrocoele moved to the light the other organs 

 lying above it rotated an equal 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 the latter seem possible, and as it may suggest itself to those who study figs. 

 G aud 7, 1 will give below the points which seem to me, directly or indirectly, to prove that the 

 hydrocoele revolves under the euterocodes and stomach, rather than that the latter twist over 

 the hydrocoele : 



(«) The ectodermal bulges, nearer the posterior end in "C" (fig. G), are the same as those 

 nearer the posterior end of " D" (fig. 9), on which the third ciliated band is situated. 



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

 the larva in both "C" and "D" (figs. G and 9), but in "C" the end toward which it points is 

 anteriorly directed in swimming and in "!>'' it points away from the end which precedes. 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 posterior end. 



(c) By any other view than the one I have adopted the blastopore, or the point where it 

 existed before closing, would be anterior and the larval organ posterior in position. In all known 

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

 occurs the larval organ originates from the anterior end of the larvae. 



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

 metamorphosis of other echinoderm larva 1 the rotation is almost entirely confined to the hydrocele. 



As the hydrocoele passes around the (esophagus the tube connecting it with the left horn of 

 the hypogastric enterocoele becomes broken aud the left anterior enteroccele, together with the 

 tube connecting it with the left horn of the hydrocoele, are carried anteriorly around the (esophagus 

 (tig. 9, ael aud st). In "D," then, we find the 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 iu "C." (Compare 

 figs. G and 7 with 9.) The anterior enterocoele comes to rest immediately in front of the stomach 

 and (esophagus.' 



From the point where the stone canal enters the anterior enterocoele the pore canal grows 

 out, passes dorsally to the ectoderm, with which latter its walls fuse, and an opening the water 

 pore (figs. 9 and 11, pc.) breaks through. Thus in this stage the cieloiu and hydrocude are first 

 connected with the exterior. 



In "0" the circular water canal had not closed, but existed in the form of a horseshoe, the 

 concave side of which opened posteriorly, but as the rotation of the hydrocude takes place its 

 horns grow toward each 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 formation of 

 which has just been described, lies between radial canals 1 aud 5 in fig. 9. 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 hydrocuele about the oesophagus, together with the growth of the ends of the 

 horseshoe, this opening 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 adradius between radial canals 5 and 1. (Compare figs. 6 

 and 9.) 



The radial canals, which existed in "C" as simple pouches from the convex side, of the hydro- 

 coele, have in ••!>" each become three-lobed. Near the tip and from the sides of each canal a pair 

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

 between and beyond them (fig. 9, et and tl). In these three structures we have the rudiments of 

 the end tentacle and the first pair of foot tentacles of the. ophiurid arm. 



When we were last considering the hypogastric enterocoele 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 



