26o A. S. PEARSE. 



bilateral and radiate stages. Correlated with the variations in 

 structure which are found in this group of animals, are corre- 

 sponding differences in locomotion, respiration, feeding and other 

 life processes, and the behavior of echinoderms is therefore a 

 matter of particular interest on account of the opportunity it 

 offers to compare the reactions of nearly related forms which 

 have somewhat different types of symmetry. The reactions of 

 certain stelleroids and echinoids (which are typically radiate in 

 structure) have been carefully studied, but no observations of 

 behavior have been made on holothurians (which are more or 

 less bilateral) except for the brief papers of Clark ('99) and 

 Grave (:o2, 105). The writer had the opportunity, during 

 July and August of the present year, of observing the common 

 sea-cucumber, Thyone briareiis (Leseur), and though lack of time 

 prevented the observations from being very extensive and left 

 many questions untested, it is believed that there are some points 

 of interest in what follows. The object of the work was {\) to 

 determine what the normal activities of this holothurian are 

 and (2) to discover how its reactions are influenced by external 

 stimulation. The experiments were carried on in the Marine 

 Biological Laboratory, at Woods Hole, Mass., and my thanks 

 are due to the director. Professor F. R. Lillie, for his kindness 

 during the work. 



II. Structural Characteristics. 

 In order that we may have clearly in mind the structural pecu- 

 liarities of the form with which we are dealing and to gain an idea 

 of the points in which it has departed from the typical radiate plan 

 of symmetry, some time will be devoted to a brief review of the 

 anatomy. Thyone briareus (Fig. i) is a spindle-shaped animal 

 which varies considerably in size, and this variation is not only 

 dependent on the age but also on the condition of contraction or 

 expansion. Fully extended individuals sometimes measure over 

 twenty centimeters in length and a specimen of this size would be 

 only six or seven centimeters long when contracted. At the pos- 

 terior end there is an opening through which water is drawn into 

 the cloaca and expelled again for purposes of respiration and ex- 

 cretion. The mouth is at the anterior end and is surrounded by 



