218 EDMUND B. WILSON. 
ment of Actinotrocha, Pilidium, and Echinoderms, and 
may have had a somewhat similar origin. The infoldings 
undergo so complex changes and become so highly special- 
ised in the insects (as in Pelidiwm) that it is not easy from 
a consideration of their development in this group alone to 
understand how they at first arose. But the simplicity of 
the process in Actinotrocha renders it possible to suppose 
that the imaginal discs of insects may be the result of the 
gradual complication of simple infoldings of the body wall, 
first acquired, as in Actinotrocha, to preserve the larval 
external form. 
The general conclusions to which a study of these pheno- 
mena of metamorphosis leads may be thus stated. Abrupt 
metamorphosis has resulted from the advantage derived 
from the abbreviation or avoidance of periods of imperfect 
adaptation. It is effected through acceleration of the 
development of certain portions of the larval organism with 
reference to a future adult state, while the remaining 
portions are specialised to meet the conditions of larval life. 
The organism thus becomes specialised simultaneously in 
two divergent directions, and in such a way that the two 
courses of development do not interfere with each other. 
The metamorphosis consists in a sudden assumption of 
functional activity by the adult set of structures and the 
simultaneous cessation or transformation of activity in the 
larval set. This may be effected either by rapidly absorbing 
or bodily casting off the larval structures or by a more or less 
complete transformation of them into adult structures ; while 
a greater or less number of larval structures may persist 
nearly unaltered as the corresponding structures of the adult. 
I gladly avail myself of this opportunity to make my 
acknowledgments to Dr. Brooks for his constant aid and 
counsel both to myself and to my fellow workers at the 
Zoological Laboratory. 
Battimore ; December, 1880, 
