OBSERVATIONS ON THYONE BRIAREUS. 263 



plan of structure, it has been modified in such a way that all of 

 the systems of organs are more or less bilateral. There are well- 

 differentiated anterior and posterior ends, but these are at the 

 extremities of an axis which is horizontal instead of vertical as in 

 most other echinoderms, for, like all holothurians, this species 

 rests on its ventral side and the oro-anal axis is hence parallel 

 to the surface on which it lies. 



III. Methods of Response to Stimuli. 

 The behavior of any animal consists of the reactions which it 

 gives in response to changes in its environment or to changes in 

 its internal condition. It is an easy matter to change the sur- 

 rounding conditions but we can interpret the internal changes 

 which an animal undergoes as it responds to various stimuli only 

 by its movements. Every species is limited to certain types of 

 response by its structure, by the medium in which it lives, and 

 by its past history. Therefore the first question to be considered 

 is, by what reactions is TJiyonc able to respond when it is stimu- 

 lated ? The chief types of response will be stated briefly and the 

 consideration of how they are brought about left for later dis- 

 cussion. 



1 . Withdrawing Reaction. — The reaction which is most often 

 seen is the withdrawal of the posterior end of the body and the 

 closing of the cloacal opening. The extent of this response de- 

 pends upon various factors and it may be so slight as to be barely 

 noticeable or it may be so marked that the body entirely disap- 

 pears beneath the sand in which the animal is buried and remains 

 out of sight for two or three minutes. Similar withdrawing re- 

 actions are performed by the anterior end of the body and also 

 by the isolated parts, such as a single tube-foot. 



2. Extending Reaction. — Under certain conditions the pos- 

 terior end of the body becomes greatly elongated and is some- 

 times stretched as much as nine centimeters above the mud in 

 which an individual lies buried. 



3. Locomotion. — If Tliyone is placed on a hard surface, such 

 as the bottom of a glass dish, it attaches the tube-feet and moves 

 across the surface and it may even climb the side of the dish. It 

 is also able to burrow into sand or mud and may move about 

 somewhat beneath the surface. 



