vol. i.] Ritter-Davis . — Enteropneusta. 195 



worth while to give them in detail. It will suffice to speak, 

 so far as concerns the experiments that resulted negatively, 

 of the methods employed. The one set of experiments that 

 seemed to yield positive results will he given more in detail. To 

 he wholly satisfactory, the following conditions of the experi- 

 ments would have to be present: (a) In some experiments at 

 least, the water would have to be exactly like that in which the 

 larvae live in nature; (h) the stages in the larval life would have 

 to be noted, and those of stages one and two would have to 

 receive particular attention: (c) the larvae would have to be 

 actively swimming: and, finally (</). the results most to be 

 depended upon would be those obtained by light admitted from 

 above in order that the swimming reaction, if induced, would 

 cause the animal to move upward. Condition <i was secured, so 

 far as could be ascertained, with reasonable exactness. There 

 was no difficulty in securing condition b excepting as to the very 

 early larval stages, but this condition may be of considerable 

 importance. The most serious difficulty was with condition c, 

 and as a result of this difficulty condition <1 was only imperfectly 

 secured. Our best efforts failed to a considerable extent in 

 keeping larvae swimming with full activity far above the bottom 

 for a sufficient length of time to carry out the experiment as 

 thoroughly as was desired. The cilia were at work constantly, 

 and seemingly with full vigor, but for some unrecognizable 

 reason there was almost always an impairment of progression. 

 Sooner or later the larvae would gravitate toward the bottom. 

 In view of the fact that it was practically impossible to keep the 

 animals constantly suspended, the effect of light on them in their 

 slow downward course was repeatedly and carefully tried. A 

 cylindrical glass jar 20 cm. in depth was arranged with one 

 vertical half darkened, and the other subjected to light of 

 various kinds and degrees of intensity. The larvae were then 

 started on the dividing line at the top, with due care, of course, 

 as to method of starting, and their tendency to either the dark 

 or light sides, in their slow course downward, noted. The result 

 of many trials of this sort showed at least as much tendency to 

 the dark side as to the light. Another series of experiments was 

 made by admitting light into the upper portion of the aquarium 



