16 



and fifty (550) Hoìotkiiria, representing twelve species, three families, 

 and five genera. One species, Thyone hriareus, was operated upon at the 

 Marine Biological Station at Woods Hole, Mass., during the months of 

 June, July, and August in 1903 ; the other eleven species were observed 

 at the Zoological Station at Naj)les from January to June, 1909. 



Since some time will elapse before the histological study of the 

 regenerated individuals can be comjDleted in all of the species observed, 

 it is thought best to publish a preliminary account of the results ob- 

 tained. In two species, Thyone hriareus and Cucumaria griibi, the 

 histology of the regenerated parts has been to a great extent worked 

 out, and the following account, deals principally with these two species. 



The complete number of species represented in the experiments 

 include the following : 



I. Dendrochirota. 

 1] Cucumaria griibi 



2) - Syricusiaua. 



3) - planci 



4) Thyone hriareus. 

 b) - fiisus. 



6) Phyllophorus urna grubi. 



II. Aspidochirota. 



1) Holothuria stellata. 



2) - spec. 



3) - impatiens. 



III. Synaptidae. 



1) Synapta digitata. 



2) - inhaerens 



3) - hispida. 



Methods. 

 In the experiments upon TJtyone hriareus one-hundred and fifty 

 individuals were operated upon. The entire anterior part of the body 

 containing the ring-nerve, was cut off and throw^n away. The anterior 

 part contained, besides the ring-nerve, the calcareous ring, which in this 

 paper will be termed the lantern; the tentacles, mouth, stomach, and 

 ring-vessels. The posterior part, containing the intestine, the repro- 

 ductive organs, and the respiratory trees, was used for the experiment. 

 In many individuals the intestine was autotomously severed near the 

 cloaca immediately after the operation. In some individuals the repro- 

 ductive organs were extruded; but the respiratory trees were in every 

 case retained. 



