MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 8i> 



(ioTO (5), too, lias shown that the hypogastric, enterocrele in the starfishes is not formed from 

 the left alone, hut in it are to be found the left and the greater part of the right posterior entero- 

 cu'les. 



The cellular plug of cells, which in "A' 1 fills the archenteric cavity, becomes divided by the 

 furrow which separates the archenteron into enteroeu'le and stomach and a part of it becomes 

 inclosed in the cavities of each of these structures (fig. 5, cp). 



STACK "('," 4X Hoi its OLD. 

 (Figures 6, 7, and X.) 



The external form of the larva, which in this series of embryos is six hours older than "B," 

 has been changed by the appearance of two lateral thickenings of the ectoderm a little posterior 

 to the median transverse plane (fig. G). 



The blastopore, which in "15" was open to the exterior, has closed, leaving no trace of its 

 former position. 



The mouth and (esophagus, which existed in "B" only as a shallow ectodermal pit, now have 

 the form of a deep, hollow tube (figs. G and 7, in and oe), which projects vertically inward until it 

 passes through the hydrowele and beyond the posterior enteroccele, when it curves back under 

 the latter to fuse with the anterior wall of the stomach. 



The stomach and posterior enteroccele are still in open communication, as in "B," but the 

 furrow in "G" has deepened, and the process by which the two structures are being separated is 

 almost complete (fig. 7). 



Although the walls of the (esophagus and stomach are fused, their cavities are still separate. 



This condition renders it easy to see just what part is played by the ectoderm in the formation 

 of the alimentary canal, the entire oesophageal cavity being surrounded by ectoderm. 



In "B" the hydroccele communicates with the posterior enteroccele by a wide opening, and 

 at the same point in " C " the two structures are still in communication, but the connection has 

 been narrowed down to a small tube (fig. 7, he). 



Beside this connection with the posterior enterocosle, a second tube has been formed, joining 

 the left anterior enteroccele with the hydroccele (fig. 7, st). This new tube, which is the rudiment 

 of the stone canal, enters the hydroca-le at the same point with the tube connecting the latter 

 with the posterior enterociele. 



The left anterior euteroco^le lies to the left of the (esophagus, and dorsal to the left half of 

 the hydrocu'le (tigs. (! and 7, ael). 



It is to be noted that, although we now have a larva possessing both hydroccele and stone 

 canal, there has been as yet no pore canal formed. This is a marked reversal in the sequence of 

 the formation of these structures from what might be expected from the order of their appearance 

 in other known echinoderms, the pore canal arising usually before the formation of the hydroccele, 

 while the stone canal appears much later than either. 



Returning to the hydrociele, we find it a horseshoe-shaped structure astride the (esophagus 

 (figs. G and 7, hy). The bulging areas which are to form the radial canals of the adult are much 

 longer and more regular in size than in "B." The radial pouch, which lies to the right of the 

 cesophagus and at the end of the right horn of the horseshoe, will hereafter be spoken of as radial 

 canal 1, since it arises from -that part of the hydroctele which was first to bud out from the 

 posterior enteroccele. The other radial canals, passing to the left over the (esophagus, will be 

 designated as 2, 3, 4, and 5. Radial canal 5 lies in this stage over the opening of the stone canal. 



The rotation of the hydrocoele around the (esophagus from its original left position, which 

 was begun in " B," has continued to such an extent in U C" that half of it lies to the right of the 

 median sagittal plane of the larva and half to the left. Radial canal 3 lies in this plane and 

 points directly toward the anterior end of the larva (fig. 6). 



With its rotation the hydroc(ele also moves bodily toward the posterior end of the larva, 

 carrying with it the (esophagus. The (esophagus, coming in contact with the anterior wall of the 

 united posterior enteroca-les, causes the latter to be pushed in at the point of contact. As the 

 process continues, those parts of the posterior euterocceles lying on either side of this in-pushing 

 area are forced to flow forward around the oesophagus and under the hydroccele; thus we have 



