150 S. O. MAST 



In his studies on Amaroucium ('20), Grave found that when 

 the larvae leave the colonies they orient and swim fairly directly 

 toward the source of light, being strongly photopositive, that 

 they remain positive for only a few moments and then become 

 negative, after which they remain so until they become attached 

 and begin to metamorphose. He maintains that quiescent in- 

 dividuals respond to sudden reduction in luminous intensity by 

 becoming active and that the eye functions as a light-receptor. 



In this paper we shall deal primarily with the nature of the 

 photic responses and the process of orientation in the larvae of 

 Amaroucium constellatum and pellucidum. 



The process of orientation has been fairly thoroughly investi- 

 gated in various organisms, ranging from relatively simple 

 plant structures to highly complicated animals, such as arthropods 

 and vertebrates. The ascidian larva, however, with but one 

 simple eye, asymmetrically located, constitutes a type in which 

 this process has heretofore received no attention. 



The results obtained in the study of the larvae of Amaroucium 

 have a direct bearing on the theories of orientation, concerning 

 which there is still much diversity of opinion. This matter will 

 be discussed briefly at the close of this paper and more exten- 

 sively in another paper dealing with the process of orientation 

 in insects. 



MATERIAL AND GENERAL BEHAVIOR 



The two species of Amaroucium which were studied are 

 sessile colonial ascidians, each colony consisting of numerous 

 zooids. The colonies during the summer months produce 

 numerous embryos which are liberated as free-swimming larvae ; 

 these become attached and develop into new colonies. The larvae 

 closely resemble amphibian tadpoles in form and they are usually 

 known as tadpoles. They consist of an ellipsoidal body somewhat 

 flattened and a broad tail about twice as long as the body. The 

 body contains among other structures a static organ and an eye. 

 The eye is located at the base of the tail near the surface of one 

 of the narrower sides of the body (fig. 1). This side or edge of 

 the body will be referred to as the ocular side and the opposite 



