88 MEMOIRS OF THE NATIONAL ACADEM? OP SCIENCES. 



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

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



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 iu thickness 

 (tig. 3). 



Cell walls are not distinguishable iu any stage of development. 



STAGE " B," 42 HOURS OLD. 

 (Figures 4 and 5.) 



According to BURY (2) the hydrowele 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 enteroccele, but iu the ophiurids it grows out from the anterior end of the left 

 posterior enteroccele. 



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

 will be seen in the description and figures of l '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 euteroca-les. The cavity of this pouch, which is the rudiment of the 

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



From the wall forming the convex sides of the hydrocwle there are, even at this early stage 

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

 ophinran (fig. 4. 1, 2, 3, 4, and ">). 



The whole hydrocu-le is curving round to the right to encircle the oesophagus, which latter is 

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

 ventral ectodermal wall (fig. 4, oe). 



To avoid confusion the hydroca'le was spoken of above as arising from the archeuteron, but, 

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

 hydroru'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 

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

 the posterior end of the posterior euterociele 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 hydrocu'le grows out, 

 hence the confirmation of BURY'S statement that it arises from the left posterior enterocce.le iu 

 ophiurids. 



In most echinoderms the posterior enterocreles originate as paired structures, and if the 

 statements of BURY and MCBRIDE are correct, that the left posterior enterocirle of the larva 

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

 the epigastric cii'lom, then, according to this, the large ventral pouch, which I regard as the 

 fused right and left posterior enterocu-les, 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 enterocu-le 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 u B " as the fused right and left posterior entero- 

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

 larval bilateral symmetry. 



