226 HELEN WOODBRIDGE. 



work, however, had shown that although on casual observation 

 larvae appeared positive to gravity at the end of the free-swim- 

 ming period, the response of the larva was not as simple as at first 

 considered. After experiments of many types had been per- 

 formed, the conclusion was reached that at the end of the free- 

 swimming period, larvae are indifferent in their response to the 

 stimulus of gravity. One experiment is sufficient to illustrate. 

 Larvae just liberated from the parent colony were allowed to meta- 

 morphose in a cylinder of 500 c. c. capacity held in an oblique 

 position at an angle of about 45 degrees. The effect of light was 

 eliminated in one case by jacketing the cylinder completely with 

 black paper; in the other three, by performing the experiments 

 in a dark-room. After the time necessary for metamorphosis 

 had elapsed the cylinders were examined. The attached zooids 

 were found evenly distributed from the top to the bottom on the 

 lower side of the cylinder in three of the four cases ; in the fourth, 

 zooids were much more frequent near the top of the cylinder than 

 the bottom, but as in the other three cases, they were much more 

 frequent on the lower than the upper side. If the larvae are 

 positive in their response to gravity at the end of the free-swim- 

 ming period, it was to be expected that the metamorphosing 

 zooids would be found attached to the lower end of the cvlinder. 



*> 



The fact that larvae were not found in large proportion at the 

 bottom of the cylinders, shows clearly that they are not positive 

 in their response to gravity at the end of the free-swimming 

 period. Their even distribution would indicate that the larvae 

 pass thru a long period of indifference to gravity during which, 

 in the absence of light, random movements send them to all parts 

 of the container, and that when metamorphic change sets in, 

 the larvae become inactive, sink, and attach themselves to the first 

 surface with which they come in contact. 



The question which naturally follows, is why, in the absence 

 of a positive response to gravity, the larvae metamorphose at 

 the bottom of a cylinder held in a vertical position, as has been 

 shown to be the case in a former paper. There are two possible 

 explanations. If the larvae attach and begin metamorphosis 

 on the surface with which they first come in contact when swim- 

 ming movements cease, as appears to be the case when confined 



