90 MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 



the posterior enteroc(ples becoming- horseshoe-shaped, the two horns of which lie under the horns 

 of the hydroccele (figs. (5 and 7, he and by). 



Lying dorsal to the stomach we find a small enteroccele which was not present in "B," or if 

 present, not in this position. It is the rudiment of the body cavity, which in the adult lies aboral 

 to the stomach and which has been recently appropriately termed the epigastric enteroccele (figs. 

 C and 7, ee). 



As to the origin of this structure I have no direct observations to give, but certain facts have 

 led me to believe that it is formed from the right anterior euterociele. These facts may be summed 

 up as follows: In "B" no epigastric enterocoele exists, but the two anterior euterocceles (fig. 4, 

 aer and ael) lie side by side anterior to the stomach and the posterior enterocudes. In " C " (figs. 

 G and 7, ee) an epigastric pouch, equal in size to the right anterior enteroccele of "B" is to be 

 found, but by the side of the (esophagus only the left anterior enterocrele remains (figs. 6 and 7, 

 ael). 



During the six hours which intervene between " B" and "C" it seems hardly possible that a 

 complete formation of the epigastric enteroccele should have taken place or that there should 

 have been time for the complete degeneration and disappearance of the right anterior pouch; 

 sufficient time may have elapsed, however, for the migration of the right anterior enteroccele to a 

 position behind the stomach. 



Against such an interpretation as the above there is the fact that in no other case has the 

 epigastric enterocn-le been observed to take its origin from the right anterior pouch. It has been 

 described as arising from the right posterior enterocu'le, however, as has been referred to before, in 

 all the groups by BURY, and his observations have been corroborated by both McBniDE and 

 GOTO in the starfishes. 



STAGE "D," 60 HOURS OLD. 



(Figures 9-14.) 



The changes which have taken place in "C" to produce "D" are very marked. 



The cilia have disappeared, except in four transverse rings or bands, three of which extend 

 entirely around the body of the larva. The third ring, counting from the anterior end, is inter- 

 rupted by the aboral disk on the ventral surface. 



This third ciliated ring first appears on the lateral bulges, which were described in "C," and 

 the fourth ring appears on a second pair of lateral bulges which originate behind the first pair 

 near the posterior end of the larva. 



The shape of the larva is no longer oval, but the posterior end has widened laterally and 

 become somewhat dorso ventrally compressed (fig. 9). The anterior end has not changed in shape 

 and may be thought of as forming the handle of the now club-shaped larva. 



The enlarged posterior end of the larva contains all its organs and is the part which will enter 

 directly into the formation of the adult ophiurid. 



From its homology with the preoral lobe and larval organ of Aster in a gibhosa I have called the 

 anterior end of the larva the larval organ. It disappears with the metamorphosis into the adult 

 form. 



The larval organ is also homologous with the stalk of the Antedon larva, although in the 

 ophiurid larva it never functions as an attachment organ. When swimming, the larval organ 

 precedes. It is filled with a network of mesenchyme cells (fig. 11, mes). 



Internally the changes have been even greater than the external ones we have just considered, 

 for it is during this period of development that the rotation and readjustment of organs takes 

 place, which is present in all echinoderms at some stage of their development. 



The hydrocu'le, which has begun its rotation about the (esophagus as an axis in "C," has 

 completed it in U D" and reached its definite position. 



That part of the hydrocu'le which in "C" was situated on the left of the plane dividing the 

 larva into bilaterally symmetrical halves, now lies on the right side of the same plane and vice 

 versa. (Compare figs. G and 9.) 



A revolution of 180 has taken place in the hydroc<ele since "C," to which if the 180 of 

 rotation be added, which took place up to the time of U C," we have a total rotation of 360 in the 



