426 PROCEEDINGS OF THE AMERICAN ACADEMY. 



As a result of such a system of reactions, Branchiostoma caribbaeum 

 falls under the bead of negatively phototropic animals, and this is also 

 the case with B. lanceolatum, which, according to W. MiiUer ('74, p. 7) 

 and others, avoids light as far as possible when in captivity, and with 

 Asymmetron lucayanum, whose habit, according to Andrews ('93, 

 p. 214), is to collect on the side of the dish away from the light. Evi- 

 dence of the same kind is also at hand for B. caribbaeum. If, into the 

 middle of a large square glass vessel so placed that the sunlight falls 

 obliquely into it through one side, living lancelets are dropped one by 

 one, they fall to the bottom as a rule without response, whereupon 

 they often begin swimming, and in practically every trial come to rest 

 near the side of the glass away from the sun. 



If a large glass aquarium is arranged so that one side and the halves 

 of the two ends adjacent to it, as well as the corresponding portion of 

 the top, are covered with light-proof paper and a number of amphioxus 

 are allowed to swim freely about in it, they will be found during the 

 day resting almost exclusively on the bottom of the darkened part, 

 whereas during the night they will be found about equally distributed 

 over the bottom. 



Since amphioxus swims away from a source of light, it is negatively 

 phototropic (Parker, :06, p. 61), and, since it is active in the light and 

 comes to rest in darkened situations, it is photokinetic (photodynamic). 



Light acts on amphioxus in a distinctly local way, and not as it 

 does on animals, like most vertebrates, which possess eyes capable of 

 forming images. This power enables a vertebrate to discriminate at a 

 distance areas of light from areas of shade in a general field. If an 

 amphioxus lying quietly in deep shade is stimulated to locomotion by a 

 minute beam of strong light, it will dart off in almost any direction 

 irrespective of the shadows and lights about it. Should it by accident 

 come into the sunlight, it usually continues to swim ; should it come 

 into shade, it usually comes to rest. The light about amphioxus has 

 little or no influence on the animal except when it falls with full 

 intensity on the animal's body. This is dependent upon the fact that 

 amphioxus is not very sensitive to light, and therefore reflected light 

 of low intensity does not stimulate it, and, further, that the light- 

 receptive organs of the animal have no adequate means for the forma- 

 tion of images. 



Under ordinary conditions amphioxus is buried in the sand, except- 

 ing for one end. Which end this is has been a matter of some dispute. 

 Yarrell ('36, p. 468) stated that the specimen from which he took his 

 description was found by Mr. Cough with its tail sticking out from 

 under a stone ; and Steiner ('86, p. 497) declared that the animal 



