92 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



anteriorly directed, the oesophagus fitted. The horns of this enteroccele, beginning in "C'to 

 grow over the hydroctnle, continue the process during the rotation of the latter, the horns of the 

 crescent growing at the expense of the thickness of its central part, ai d in ''D" we have this 

 enteroctele lying directly over the hydroc(ele in the form of a perfect horseshoe (fig. 9, he). 



Between tiie ends of tlie horns of the hypogastric c(flom lies the anterior enteroc<ele. The 

 walls of these structures come together and fuse in such a way that tliey together form a hollow 

 circular ccelom surrounding the stomach and lying over the somewhat smaller water vascular ring 

 (fig. 9). 



In the four interradii, marked by their positions between radial canals 1 and -', 2 and .'?, 3 and 

 4, and 4 and .">, four jiouches of the hypogastric enterociele grow downward, outside the water 

 vascular ring, forcing themselves between the radial canals; a fifth pouch, similar to those Just 

 described, is formed from the left anterior enterocoi-le in the remaining interradius between radial 

 canals 5 and 1 (figs, t), lli, 13, and It, hip 1-2, 2-3, 3—1, 4-5, and ipax 5-1). These five pouches 

 are the rudiments of the outer periba'mal ring, which will be more fully considered in the 

 succeeding stages. 



The stomach, after being entirely cut off from the hypogastric enterocd'le, was drawn forward 

 during the rotation of the hydrocccle, and the oesophagus was carried in the opposite direction, 

 so that in "D" the stomach lies almost directly over the tesophagus (figs. 8 and 13, oe and s). The 

 partition, which in "C" separated the cavities of these two strnctures, has disappeared in '-D," 

 and the (esophageal cavity opens into that of the stomach. There is present, then, in "D" the 

 definitive alimentary canal of the adult ophiurid. 



The "cellular mass," which in "B" and "C" Avas being divided into two parts by the 

 constriction separating the archenteron into enteroctcle and stomach, is to be found, in sections 

 of "D," in the cavities of both the above structures (figs. 11, 12, 13, and 14 cp). 



Lying immediately above, or aboral to, the stomach is to be found the epigastric entcroco'le. 

 It has eidarged considerably during the interval between "C" and "D,'' but is not yet of sutli(;icnt 

 size for its walls to touch those of the hypogastric ccelom, and hence in this stage no circular 

 aboral mesentery is to be found. 



Stack "E," (>6 Hours Oli>. 

 (FiiEcures 15-21.) 



The thickening of the ventral ectoderm which was begun in "l)''has continued during tlie 

 six hours which intervene between "D" and "E" and has spread to the sides of tlie larva (figs. 

 15 and 19-21). 



Near the edge of this thickened oral disk are to be found five groujis of rounded elevations of 

 the ectoderm (fig. 15, 1, 11, III, IV, and V). The three elevations, of which each grouj) consists, 

 form the angles of an isosceles triangle the apex of whicli points away from the moutli of the 

 larva (tig. 15). These elevations or evaginated papilla' lie immediately below and inclose tiie tilts 

 of those branches of the radial water canals which form tlie rudiments of the end tentacle and 

 first pair of foot tentacles of each arm (figs. 1!) and 21). In this way each tentacle grows into its 

 ecto<lerm, the latter closing around it as it pushes out. 



The function of these tentacles in the adult being mainly a sensory one, it is interesting to 

 note that they receive their ectoderm from jtart of the same thickened oral area which gives rise 

 later to the adult nervous system. 



The ciliated bands in "Iv' do not difl'er in appearance and position from those in "D," but 

 since they wen- not figured in the earlier stage it may be well to refer to them again in connection 

 with figs. 15 and 10, cb 1, 2, 3, and 4. The first or most anterior band surrounds the larval organ 

 quite near its tip. 



Near the first baud, and itarallel to it, runs the second one also around the larvid organ. The 

 third ciliated band is separated from the second by a much wider space than that which separates 

 the first and second bands. Were it not interrupted on the oral disk the third ciliated band would 

 lie in the line separating the bivium and trivium — that is, between arms 1 and II on the one side 

 and IV and ^' on the other. The fourth band, passing just posterior to tlie group of ectodermal 

 elevations lying under the branches of the thiril radial water tube, surrounds the posterior end of 

 the larva. 



