PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 641 



In a later stage, at which the arms are branching, the epithelium of the oral surface 

 has become thin, in marked contrast to the stage before the opening of the vestibule. 

 The mouth may protrude above the oral surface like a small funnel so as to appear 

 in transverse sections as a ring. In one case Mortensen found a thickening of the 

 esophageal wall in the posterior interradius which made a distinctly delimited furrow 

 that ultimately closed into a narrow canal. The thickening began at the level of the 

 hydrocoele ring and continued for some distance below the rectum, ending as a small 

 ridge that rose into the lumen of the stomach. Mortensen was at a loss to explain 

 this structure, which may, he said, be some abnormality perhaps due to the fact that 

 the stomach in this individual was strongly distended by the embryo which it had eaten. 

 In another case there was a thickening of the esophageal wall in the posterior inter- 

 radius; but in this in passing downsvard it soon broadened and passed gradually over 

 into the normal condition of the entodermal epithelium. 



The stomach is provided with folds. In the strongly dilated stomach of the 

 individual that had eaten an embryo these folds had disappeared. 



The anal opening has been formed. 



The hydrocoele ring is not yet completely closed; numerous trabeculae have been 

 formed in its lumen. A thickening of the epithelium over the upper side of the hydro- 

 coele ring is the rudiment of the nerve ring; below this is an indication of circular muscle 

 fibers. The stone canal has lengthened considerably, and the pore canal has again 

 acquired an opening to the exterior. The parietal canal has opened into the oral 

 coelome. In the aboral coelome some irregularly disposed trabeculae are seen. 



The "primary gonad" is distinct. 



The axial organ has developed into a conspicuous cord, and the chambered organ 

 is very distinctly quinquelocular with a fairly large lumen. 



In the arms of the larger pentacrinoids the primary (azygous) tentacle is seen 

 situated in the fork. As it remains here it must ultimately be resorbed. The radial 

 canals of the arm branches originate as lateral offshoots from the primary radial canal 

 a little below the free azygous tentacle. In each triplet of tentacles the distal is the 

 first to develop. 



At the first stage in the development of the skeleton the orals and basals, as 

 branching spicules, are arranged in two circlets, the plates of the basal circlet lying 

 almost exactly below the corresponding plates of the oral circlet. The terminal stem 

 plate has been formed, and a few columnals are indicated by very small spicules. 

 Infrabasals are wholly lacking. 



In the fully formed larva the orals and basals are large fenestrated plates arranged 

 in 2 regular semicircles leaving a broad open gap on the ventral side. The two semi- 

 circles are at first widely separated from each other, the orals occupj^ng the posterior 

 and the basals the anterior end of the larva. The terminal stem plate is large and 

 fenestrated, and the columnals have increased considerably in size and number. Their 

 exact number is difficult to ascertain because they lie so close together. 



Finally, the oral and basal plates increase in size until they are in contact with each 

 other and reach to near both the poles. This stage was- observed by Dr. John in the 

 Discovery material. 



The freshly attached pentacrinoid is very completely enclosed by the orals and 

 basals. The columnals are still very short, and so closely crowded that it is impossible 

 to count them. The proximal third of the column is still enclosed by the basals. 



