MONOGRAPH OF THE EXISTING CRINOIDS. 421 



of the visceral basin and assist in supporting the visceral mass. It will be seen 

 that the line of tear forms the edge of the visceral basin. This edge does not main- 

 tain an even curvature all the way around but is deeply notched. There are 10 of 

 these notches or bays, one between the two arms of every pair, and one between 

 each two adjacent arm pairs, the latter being a good deal the deeper. 



The floor of the visceral basin is lined with an exceedingly thin transparent 

 membrane of connective tissue through which the muscles between the calcareous 

 ossicles are very distinctly visible. The degree of distinctness with which these 

 muscles are visible beneath the overlying regenerating tissues forms, during the 

 earliest stages, a good indication of the amount of regeneration which has taken 

 place. 



By comparing sections of an uninjured specimen and sections of an isolated 

 visceral mass it is found that the separation of the visceral mass takes place 

 between two layers of connective tissue, one lining the visceral cavity, the other 

 covering the aboral surface of the visceral mass. 



Forty-three hours after evisceration specimens of Antedon bifida exhibit a 

 thickening of the connective tissue lining of the floor of the visceral basin. The 

 interarticular muscles are much less distinct, though still plainly visible. The 

 thickening is more marked around the edges of the injured area than in the center, 

 so that the muscles between the radials and IB^ are much more distinctly visible 

 than those between the second primibachs and the IBr v In one specimen of this 

 stage Dendy found the outer circle of muscles nearly hidden by the ingrowing 

 mass of connective tissue. The line of tear along which the visceral mass has been 

 separated from the calyx no longer appears sharp and distinct. 



At two days the new growth of connective tissue, which had commenced 

 at 43 hours, has proceeded farther. The muscles on the floor of the visceral basin 

 are all completely hidden by the newly formed tissues. The line of tear is marked 

 by the sudden stopping of the lappets and pigment spots along the borders of the 

 ambulacral grooves. In sections it is seen that the surface of the regenerating 

 cushion of tissue is covered by a thin, deeply staining layer, which is apparently 

 formed by the arching over and ingrowth of the edges of the injured area. This 

 layer is formed of two parts an upper, which is probably epidermal, and a lower 

 dermal. The deeper connective tissue layer is continuous with strands of the 

 same substance which have grown out from the floor of the visceral basin. In 

 the center, above the chambered organ, the superficial layer of this regenerating 

 visceral mass is still incomplete, so that on looking at it from above two or three 

 little holes are visible in the center leading down to the chambered organ. The 

 regenerated cushion of tissue is still very thin, averaging in thickness only about 

 half a millimeter. 



Regeneration, then, appears to commence in two chief ways (1) by a series 

 of outgrowths from the thin layer of connective tissue which forms the floor of the 

 visceral basin, and (2) by an ingrowth of connective tissue and epidermis from 

 the edges of the injured area forming a roof to the visceral basin. 



At the next stage the new visceral mass is seen to have grown and has thick- 

 ened considerably. The openings leading down to the chambered organ are now 



