88 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



archenteron is formed by a splitting away of the central core. Ill the same way the plug of cells 

 is probably formed by the hollowing out of the solid archenterou. 



Beside their ragged outline the walls of the larva have another peculiarity in their structure, 

 for, judging by the number and position of the nuclei, they are from two to three cells in thickness 

 (fig. 3). 



Cell walls are not distinguishable in any stage of development. 



STAGE " B," 42 HOURS OLD. 

 (Figures 4 and 5.) 



According to BURY (2) the hydrocoele does not have the same origin in all the groups- of 

 echinoderms. lie found that it originates in the crinoids, sea-urchins, and starfishes from 

 the left anterior enteroctele, but in the ophiurids it grows out from the anterior end of the left 

 posterior enterocoile. 



This observation, which BURY records with apparent hesitation, I can completely confirm, as 

 will be seen in the description and figures of U B." 



Externally the appearance is the same as in "A," but the internal structures have undergone 

 a great change. 



The anterior pouches, the cavities of which in "A" were connected both with each other and 

 with the cavity of the archenteron, are now separate and distinct. The connection between these 

 structures still continues, however, in their fused walls. The left pouch is a little larger than the 

 right and lies behind and to the left of the latter (fig. 4, ael). 



Just below the anterior pouches there is to be found a third pouch, which is growing out from 

 the left side and anterior end of the archenteron (fig. 4, hy). It protrudes anteriorly and partially 

 covers the two anterior enterocu j les. The cavity of this pouch, which is the rudiment of the 

 hydroco'le, is in wide communication with the archenteron. 



From the wall forming the convex sides of the hydroco?le there are, even at this early stage 

 in its formation, five outgrowths which are the beginnings of the radial canals of the adult 

 ophiuran (fig. 4. 1, 2, 3, 4, and 5). 



The whole hydrocu j le is curving round to the right to encircle the o?sophagus, which latter is 

 making its first appearance in this stage as a shallow but definite pit in the central part of the 

 ventral ectodermal wall (fig. 4, oe). 



To avoid confusion the hydroccele was spoken of above as arising from the archenteron, but, 

 as will be seen in the transverse section (tig. 5), taken in a plane posterior to the origin of the 

 hydroriL'le, a differentiation is taking place in the archenteron which enables us to distinguish in 

 it the rudiments of two structures, the posterior enterocceles and the stomach. By a longitudinal 

 circular furrow the archenteron is being cut horizontally into a large ventral pouch, the posterior 

 enterocceles (pe) and a smaller dorsal one, the stomach (s). This stomach rudiment bends around 

 the posterior end of the posterior euteroccele and opens to the exterior through the blastopore 



(fig- 4). 



It is from the left side and anterior end of the ventral pouch that the hydrociele grows out, 

 hence the confirmation of BURY'S statement that it arises from the left posterior enterocffile in 

 ophiurids. 



In most I'chiuoderms the posterior euterocoales originate as paired structures, and if the 

 statements of BURY and McBRiDE are correct, that the left posterior eiiteroctele of the larva 

 forms the hypogastric body cavity of the adult, and the right posterior enterocit-le goes to form 

 the epigastric ccelom, then, according to this, the large ventral pouch, which I regard as the 

 fused right and left posterior enteroctples, really represents the left only, because it takes no part 

 in the formation of the epigastric body cavity of the adult ophiurid, but, with the left, does pass 

 directly into the hypogastric. 



The origin of the epigastric enterocojle is discussed in the description of Stage " C," in which 

 its rudiment is first found over the stomach. 



My reason for regarding the ventral pouch of " B " as the fused right and left posterior entero- 

 cieles, is that at the time of its origin it is symmetrically disposed on either side of the plane of 

 larval bilateral symmetry. 



