426 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



ccelomic canals, which, so far as can be seen, contain very few cell elements, for in 

 general the contents of these cavities are extraordinarily poor in lymph cells and 

 similar structures. This fluid does not coagulate. 



The ventral epithelium, together with the underlying cutis, now degenerates 

 with extreme rapidity and, by spreading over the openings of the water and coelomic 

 vessels, closes the wound, this closure being in many cases assisted by elements 

 emerging from the canal containing the dorsal (mesodermal) nervous system. 

 All the other brachial elements, such as the calcareous structures and the surfaces 

 of the ligaments and muscles, remain uncovered. 



The course of the degeneration is entirely different in the different tissues. It 

 is commonly most extensive in the epithelial elements, where the most specialized 

 cells are first affected, the epithelial gland cells, the nervous elements, the sup- 

 porting cells, etc., until finally a covering of loose fragments, granules, and fibers, 

 which all fuse together, is formed. 



The degeneration of the water vessel is usually not extensive, nor is that of 

 the coelomic canals and their epithelium. The most far-reaching degeneration 

 is that experienced by the dorsal nerve cord, which, however, heals more rapidly 

 than any of the other structures. 



In most cases degeneration commences without the intervention of phagocytes, 

 and appears to result from the stimulus of the wound itself. 



There are two types of cells which function especially in degeneration and 

 regeneration. The first of these are the lymph cells or phagocytes, which are 

 present in all crinoids, though not especially numerous. They concentrate on, 

 or in the vicinity of, places undergoing rapid degeneration, breaking down the 

 injured tissues and carrying the residue away. The particles, bits of dorsal liga- 

 ment or muscle, or of other tissues, are surrounded with pseudopodialike processes 

 and gradually removed into the interior of the animal there to undergo dissolution. 

 This takes place rather .slowly. Sometimes the phagocytes intrude into entirely 

 sound tissue in the vicinity of the wound and there cause histolysis. 



The cells of the second type are the wandering cells which accompany the 

 dorsal nerve cord and its brandies in enormous numbers. As soon as a fracture 

 occurs a great migration of these cells toward the wound begins. They elongate 

 into wormlike shapes and stream along by the shortest possible path. At this time 

 the wandering cells are very characteristic; they are more or less filled with a 

 widely varying number of dark granules and little rods. That they come from 

 a long distance is evident from the fact that, in spite of the lateral migration out 

 of the dorsal canal over the surface of the wound, no perceptible gaps appear 

 in the investment of the nerve in the mutilated arm. 



There appears to be a definite division of labor between the phagocytes and 

 the wandering cells, the former being occupied principally in removing old tissue, 

 the latter in bringing up new material for the regenerative processes. 



RehuUding of the \imter hike and of the coslom-ic can-oL—The degeneration 

 of the water tube is very limited in extent. After it has remained closed for a 

 longer or shorter period, depending on the seriousness of the injury, a little cap 

 appears which at the most consists of three compact layers of cells, often of only 



