MONOGRAPH OF THE EXISTING CRINOIDS. 423 



inner glandular layer of its walls being formed from invaginated epidermis, and the 

 outer connective tissue layer from the underlying dermis. 



At five days the condition is about the same; tlie alimentary canal is very 

 small and almost solid, the inner layer of its walls being composed of minute 

 deeply staining nucleated cells, more or less rounded in shape and showing a slight 

 tendency to become columnar in places, and being histologically indistinguishable 

 from the tissue covering the general surface of the disk. Here, again, the ali- 

 mentary canal appears to have been formed by invagination, accompanied by rapid 

 proliferation of the cells of the epidermis. In the specimen described this pro- 

 liferation has formed a thick mass of minute cells projecting on the surface at one 

 side of the mouth. No anal cone is yet visible. The terminal portion of the in- 

 testine touches the roof of the visceral mass, but no definite opening was found. No 

 ambulacral nerves or canals are as yet visible around the mouth. A number of 

 new ambulacral pores have been formed, apparently hj invagination; the epi- 

 thelium lining them is at first not distinctly columnar. 



At nine days the boundaries of the ambulacral grooves are seen to have met 

 and formed a pentagon around the mouth. They inclose a slightly depressed area, 

 with the mouth in the center, and appear as thickish white ridges, in some places 

 notched, showing where lappets are begimiing to regenerate. 



Dendy believes that the ambidacral grooves are, from the first commencement 

 of regeneration, left as areas along which the thickening of the regenerating tissue 

 is not so great as elsewhere. This thickening takes place centripetally in five dis- 

 tinct areas, one in each interradius. As these grow inward the ambulacral grooves 

 and the central depression around the mouth are left as less thickened portions. 

 The mouth is oval, runs obliquely downward, and is now for the first time dis- 

 tinctly bounded. 



The beginning of the anal cone is visible as a small conical papilla placed inter- 

 radially. In sections the alimentary canal is seen to be fairly well developed; it 

 contains food refuse, indicating that its use has begun. 



The epidermis covering the depressed areas around the mouth is now com- 

 posed of colimanar nucleated cells and passes gradually into the inner lining of the 

 esophagus, the cells of which are still more columnar. 



The walls of the stomach are also composed of the usual two layers — an inner 

 layer of columnar nucleated cells and an outer layer of connective tissue. This 

 outer layer is connected with the loose connective tissue filling the body ca\'ity. 

 In the specimen described the cells forming the inner wall of the alimentry canal 

 have become columnar right up to the anus. The anus is very minute and is situ- 

 ated on the top of a small papilla. The columnal cells lining the cavity of the 

 anus are very small and pass gradually into the epidermal cells covering the rudi- 

 mentary anal zone ; the epidermal layer at this point is thickened. In the region of 

 the stomach the walls of the alimentary canal are beginning to become folded, 

 especially on the axial side, and on this side also the glandular layer is thicker. 



In another specimen of this age, which appears not to have advanced quite so 

 far in regeneration, the alimentary canal, as it approaches the anus, narrows very 

 much. Its inner wall, which in the first part of its course is composed of the usual 

 142140— 21— Bull. 82 29 



