BOTRYLLUS SCHLOSSERI (PALLAS). 22Q 



Do these various reactions the responses to light and gravity, 

 and the " habit " of swimming upward when resuming activity 

 or when coming in contact with a perpendicular surface have 

 an adaptive value to the species? When considered in relation 

 to the natural habitat, they assume considerable significance. 

 Adult colonies of Botrylliis are attached very commonly to eel 

 grass, or other objects projecting above the bottom, such as the 

 submerged parts of floats, or rockweed. They are always found 

 below low tide mark, and never on the bottom, being found only 

 on the parts of eel grass blades which are not exposed at low 

 tide, and the submerged parts of logs. Sand or mud on which 

 the eel grass thrives offers no place for attachment and meta- 

 morphosis to the larva. It is evident from a study of the dis- 

 tribution of the adults that those larvae which attach too near the 

 surface or which sink to the bottom do not reach the adult con- 

 dition. The responses of the larva are such as to keep it away 

 from these unfavorable environments Xhe positive response 

 to light and the negative response to gravity at the beginning 

 of the free-swimming period serve to bring the larva to the sur- 

 face and to distribute it more widely than would be probable 

 without these responses. There follows a period of indifference 

 when the larvae show random movements ; they do not orient 

 to stimuli either of light or gravity. During this period the 

 adaptive value of the response of swimming upward when re- 

 suming activity, or when coming in contact with a surface is 

 evident. The negative response to light, were it unmodified by 

 these additional responses might force the larva at the time of 

 metamorphosis into a position too near the bottom. The re- 

 sponse of swimming upward keeps it away from the bottom, 

 and tends also to keep the larva in proximity to objects suitable 

 for attachment. In experiments such as were first described, 

 in which larvae werex allowed to metamorphose in crystallization 

 dishes containing eel grass blades stretched obliquely from the 

 bottom of one side to the top of the other, larvae were many 

 times observed to be trapped by these responses in the region of 

 the grass blades. Larvae sinking motionless thru the water, 

 frequently came within range of shadows cast by the grass. 

 Instantly they would resume activity in an upward direction. 



