90 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



the posterior enterocMi-les becoming borseslioesbaped, tbe two bonis of wbicb lie uuder the horns 

 of the byilrocd'le (figs. and 7, be and by). 



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

 present, not in this ])osition. It is tbe rudiment of tbe body cavity, wbicb in tbe adult lies aboral 

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

 G and 7, ee). 



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

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

 up as follows: lu "B" no epigastric enteroc(ele exists, but the two anterior enterocceles (fig. 4, 

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

 G and 7, ee) an epigastric pouch, equal in size to tbe right anterior enteroc<ele of "B"' is to be 

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

 ael). 



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

 complete formation of tbe epigastric enteroetele should have taken place or that there should 

 have been time for tbe complete degeneration and disappearance of tbe rigbt anterior jioucb; 

 sufficient time may have elapsed, however, for the migration of the right anterior eiiterocu?le to a 

 position behind tbe stomacb. 



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

 epigastric enterociele been observed to take its origin from tbe right anterior pouch. It has been 

 described as arising fiom tbe right poxtrrior enteroctelc, however, as has been referred to before, in 

 all the groups by Burv, and his observations have b»'en corroborated by both Mcl}Ri]>E and 

 Goto in tbe starfishes. 



Stage "D,"fiO Houns Old. 



(Figures !l-ll.) 



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



Tbe cilia liave disappeared, except in four transverse rings or bauds, three of which extend 

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

 rupted by tbe aboral disk on the ventral surface. 



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

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

 near tbe posterior end of the larva. 



Tbe shape of tbe larva is no longer oval, but tbe posterior end has widened laterally and 

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

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



Tbe enlarged jjosterior end of the larva contains all its organs and is tbe part which will enter 

 directly into tbe formation of the adult ophiurid. 



From its homology with the preoral lobe and larval organ of Asterhia gibbom I have called tbe 

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

 form. 



Tbe larval organ is also bomologous with the stalk of the Antedon larva, although in tbe 

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

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

 jdace, which is i)resent in all echinoderms at some stage of their development. 



Tbe bydroc(cle, wbicb has begun its rotation about tbe (esophagus as an axis in "C," has 

 completed it in "D" and reached its definite i)osition. 



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

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

 versa. (Compare figs, (i and 9.) 



A revolution of 1S(P has taken i)lace in the bydrocielc since "C," to which if tlie lS()o of 

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



