286 A. S. PEARSE. 



X. General Considerations. 



Tliyonc hriareus is a holothurian which is rather strikingly 

 adapted to a sedentary life. It is not able to change its place of 

 abode easily and it is hence highly resistant to unfavorable con- 

 ditions in its environment. Individuals which were allowed to 

 lie on moist sand exposed to the air for eighteen hours were 

 apparently uninjured. This tenacity to life is also shown by the 

 ability this species manifests to withstand changes in the temper- 

 ature and the density of the water in which it lives. The 

 methods of feeding, locomotion, respiration and other activities 

 are adapted to the peculiar conditions under which it exists. 

 Passing most of its life buried in the mud, Thyone probably does 

 not often fall a prey to large enemies but it is protected from 

 them by the withdrawing reaction, by its locomotion aw^ay from 

 the light and by its habit of pulling pieces of eel grass and other 

 debris over the body. 



Many of TJiyone's movements show a lack of correlation. In 

 ordinary locomotion on a solid surface, the tube-feet which are 

 behind are often forcibly pulled loose from their attachments 

 instead of being released by means of some impulse from the 

 central nervous system. Such organs as the tube-feet are able 

 to work more or less independently, but they may also be actu- 

 ated by a unified impulse, as is shown when they are simultane- 

 ously extended or contracted over the whole body and the same 

 unity is apparent in their action as they pull the animal along in 

 a definite direction. On the other hand many reactions show 

 considerable power of correlation and adaptation. Correlation 

 is shown in the use of the circum-oral tentacles, as they move in 

 a rather definite order. Very often, however, two of them 

 endeavor to enter the mouth at the same time, but one always 

 bends aside to make way for the other. If the correlation in the 

 movements was perfect in this case, two tentacles would not try 

 to enter the mouth at once, and if there was no correlation they 

 w^ould struggle with each other indefinitely. Furthermore, when 

 a feeding individual lies on its side, the tentacles which scrape 

 the bottom are used oftener than the others and there is thus an 

 increased chance of obtaining food. 



Generally speaking, it may be said that Thyone s hehavior, 



