154 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



usually either lies entirely dormant, or manifests itself in 

 its completed and typical condition. 



B. Specific Characters. 



The characters that have been used to differentiate the 

 species of the genus Botrylhts are in the main: — 



1. Thickness of the colony, and the ease with which it 

 is detached from its substratum. 



2. The size of the systems, and the intervals separating 

 them from each other. 



3. The size of the colonies and zooids. 



4. The color of zooids and ampulla. 



I. Thickness of the Colony. 



The thickness of the colony is of much value in dis- 

 tinguishing the various genera of the Botryllidae ; but, within 

 the genus Botryllus, the comparatively small variations 

 observed were not congenital, nor even constant, for the 

 same colony. The thickness varied in the same colony 

 with its age and environment; and the same must be said 

 of the ease with which the colony is detached. Thus, all 

 the young and vigorous colonies that have plenty of room 

 in which to grow, and a clean substratum, are thin, and 

 are firmly attached to the substratum. The older colonies 

 that are not growing so fast are thicker, even when they 

 have plenty of room in which to spread; and when their 

 area of attachment is limited by competition with other 

 incrusting organisms they are thicker still. The extreme 

 in this direction was seen in a Woods Hole colony which 

 was being encroached upon from all sides by an incrusting 

 bryozoan. Since it could not spread out laterally, it had 

 grown in thickness until it had become a little sphere 

 about one centimeter in diameter. 



The ease with which a colony can be detached depends, 

 so far as could be seen, entirely upon external conditions. 

 A firm attachment is only possible when the substratum is 



