INTRAOELLULAK DIGESTION OP INVERTEBRATES. 97 



be seen at two stages in the life-history of Auricularia. They 

 first occur at the assumption of the so-called pupa stage, 

 when a large part of the longitudinal ring of cilia is lost — that 

 is, is disintegrated and devoured by the mesoderm. At this 

 time every amoeboid cell of the middle layer is generally 

 loaded with enormous numbers of debris granules, which are 

 slowly absorbed during the pupa stage, so that the cells which 

 contained them become filled with clear vacuoles. On the 

 metamorphosis of the pupa into a young Synapta, the cells 

 begin again their devouring work, collecting as before beneath 

 the ciliated rings, and eating up the products of disintegra- 

 tion. The appearances seen during the process at this time 

 are exactly those seen at the time of its first occurrence. 



Similar phenomena can be observed in Asterid larvae^ where 

 large tracts of larval tissue atrophy during the metamorphosis. 

 In this case, also, the disintegrating cells break up into albu- 

 minoid granules of various sizes, which are gradually eaten 

 and absorbed by mesodermal elements. 



I have found these appearances so constantly to accompany 

 metamorphosis, that I cannot but regard them as normal and 

 necessary events in the life of an Echinoderm larva; and 

 I am therefore forced to the conclusion that the wandering 

 mesoderm cells of which I have spoken play the same part 

 in the resorption of larval organs as that played by osteoclasts 

 in the resorption of Vertebrate bone. 1 have never seen, 

 however, in Echinoderm larvae, any formation of multinuclear 

 masses similar to those seen during the resorption of bone. 



It is hardly possible to believe that this resorbent function 

 of the mesoderm should be confined to Echinoderms. I rather 

 incline to the belief that it occurs in all animals whatever which 

 undergo any great degree of metamorphosis. I have reason to 

 believe that wandering cells play an essential part in the com- 

 plicated larval changes of Ascidians. I have not, indeed, been 

 able to prove this in the case of Ascidia intestinalis ; but 

 only, I believe, because of the small size of the cells in 

 question; but I have frequently seen wandering cells loaded 

 with debris. If this should prove to be the case, we should have 



VOL. XXIV. NEW SER. G 



