OBSERVATIONS OF THYONE BRIAREUS. 287 



like that of other sedentary animals, is mostly made up of stereo- 

 typed reactions which occur regularly in response to certain 

 stimuli. Furthermore, many of these reactions are carried on 

 independently by certain separate organs and two parts of the 

 body may "work against each other" for a time, but, under the 

 proper conditions of stimulation, all these simpler reactions may 

 be unified into one general correlated response. Although the 

 reactions are largely stereotyped in nature they may be changed 

 by experience or inhibited by the presence of different stimuli. 

 The stereotyped methods of response are usually adequate to 

 meet the conditions under which JJiyone exists and would usually 

 enable it to survive in the struggle for existence. If they are 

 not adequate, however, they may be modified to meet new con- 

 ditions. For example, this species usually burrows into the 

 mud so that only the posterior tip of the body is exposed and 

 even this is withdrawn if the slightest shadow falls upon it or if 

 the water is agitated. If the water becomes stagnant the same 

 individuals that were formerly so reactive will climb up the side 

 of the jar and cease to respond to such slight stimuli as shadows 

 and water currents, and they contract only when touched by 

 some solid object. As the water becomes foul, the greatest 

 need of the organism is oxygen and the behavior described would 

 enable this to be obtained, but to accomplish this end, the 

 animal would be obliged to forego the temporarily less important 

 matter of protection from its enemies. 



When compared with an echinoid or a star-fish as described by 

 Romanes ('85) and Jennings (:o7), or with an ophurian as it is 

 represented by Glaser (:07) T/iyone falls short in the range and 

 diversity of its reactions. This is probably due in part to its 

 sedentary mode of existence and the study of holothurians which 

 do not burrow might show a somewhat different set of reactions. 

 Perhaps the most interesting point which is brought out in the 

 study of 774;/(?;/^'.y behavior is the fact that, although the symmetry 

 is so strikingly bilateral, the locomotion is carried on with the 

 same lack of orientation which is so characteristic of other groups 

 of echinoderms. 



