MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 93 



In "E" the cavities of the oesophagus and stomach have become obliterated, and the two 

 structures appear in section as one solid mass of cells (fig. 20, st and oe). No degeneration in 

 their size, however, is to be observed, and their outer walls remain well defined, the oesophagus 

 retaining its connection with the ectoderm. As will be seen later, their lumen reappear and they 

 become the definitive alimentary organs of the adult ophiurid. 



Returning to the consideration of the water system, we find in "E" instead of one pair of 

 tentacles on each radial canal, as in " 1),'' there are two pairs present (fig. 17, tl and t2), the 

 second pair having grown out of the radial canal between the first pair and the water ring. The 

 second pair is much smaller than those which were first to be formed, and, contrary to what 

 one would expect, this discrepancy in size does not disappear as time goes on. This is also true 

 in the sea-urchins, in which the primary tube feet in the larva are enormously larger than those 

 which are subsequently formed. The primary tube feet in this case gradually diminish in size 

 after the adult form is reached. 



As a rule, among echiuoderms the tube feet or tentacles are formed centrifuyally from the 

 radial canals; that is, between the end tentacle and the last pair of tube feet or tentacles already 

 formed. This process keeps the undiffereutiated growing point of each arm at its tip, but in this 

 ophiuran, and the same is true of Antedou, the formation of the tentacles begins in a centripetal 

 manner; that is, the second pair of tentacles appears, not between the end tentacle and the first 

 pair, but between the first pair and the ring canal. 



This second pair of tentacles is the rudiment of the buccal tentacles, and although differing in 

 both function and position in the adult from that of the foot tentacles, is nevertheless entirely 

 homologous with the latter. This homology is shown by their origin and the fact that for a time 

 after forming they are directed away from the mouth toward the end of the arm just as is the case 

 with the foot tentacles. After a time, however, as will be seen later, they turn back and point 

 toward the mouth, thus showing that in this second pair of outgrowths from the radial canals 

 we have to do with the first pair of buccal tentacles of the adult. After budding, as we see, from 

 the radial canals, they migrate to a position on the ring canal, with which we find them connected 

 in the adult. 



In " E" the buccal tentacles have no ectoderm nor rudiment of such, the ectoderm under their 

 tips being as yet undiffereutiated from the oral disk. 



The hypogastric enterocoele has assumed a more pentagonal shape than in "D," it having 

 grown out over the radial water canals (fig. 17, he). These projections of the hypogastric eutero- 

 ccele will continue to grow with the growth of the arms and become the brachial extensions of 

 the body ccelom. 



The interradial pouches of the hypogastric enteroccele, which were beginning to form in "D," 

 have pushed down further and further between the radial canals until, coming in contact with the 

 ventral ectoderm, they bend over, inserting themselves between the ring canal and the oral disk 

 (figs. 17-21, hip 1-2, 2-3, 3-4, and 4-5). In the same way the pouch from the anterior enteroctele in 

 the stone canal interradius has grown under the water ring. In these five interradial enterocffilic 

 outgrowths, as has been mentioned before, we have the rudiments of the outer perihremal sinus 

 of the adult. The process by which this periLuernal sinus is formed in Ophiura ln-evispina agrees 

 in every detail with its method of origin in Asterina gibbosa as described by McBuiDE (10). 



The epigastric euterocffle is in much the same condition as that in which we left it in ; 'D," it 

 being as yet too small to meet and form a mesentery with the dorsal edges of the hypogastric 

 co^lom (fig. 19, ee). 



The stone and pore canals, too, have changed very little during the interval between "D" 

 and " E." From the ring canal at a point to the left of the origin of radial canal 1 the stone canal 

 passes upward and opens into the right postero-dorsal part of the anterior enterocode. The pore 

 canal begins at the same point where the stone canal ends, the two canals thus having a common 

 opening into the anterior euterocosle or ampulla}. The pore canal extends from the enterocoale to 

 the dorsal surface of the larva, where it empties through the dorsal pore at a point a little to the 

 right of the median sagittal plane. These two canals, although extending in the same direction, 

 do not lie in the same straight line, the pore canal being set a little anterior to and to the right of 

 the stone canal (figs. 17, IS, and 21, st and pc; also fig. 11). 

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