2 2 WILSON. [Vol. II. 



contractile power that it frequently tears the umbrella cells 

 apart by violent contractions when irritated. 



While floating about freely these mesenchyme cells do not 

 contract, so far as could be observed, but as soon as they begin 

 to form processes they can be seen to contract. Occasionally 

 two cells anastomose with each other before becoming attached 

 to the wall of the pilidium. 



In such a case they contract irregularly at first, the intervals 

 between contractions being unequal, but later the contractions 

 become rhythmical and very closely resemble those of the 

 opisthobranch gastropods just described. Three or four con- 

 tractions occur in rapid succession and are followed by a 

 comparatively long rest. After the cell branches become 

 fastened to the pilidium wall these rhythmical pulsations cease. 

 The mesenchyme cells now become regular muscles of the larva 

 and contract only when stimulated from the central nervous 

 system. 



We are witnessing here, then, the passage from an automatic 

 condition, in which the cells contract quite independently from 

 the rest of the larva, into a condition in which every contraction 

 is definitely correlated with that of the other larval muscles. 



In this nemertean larva the intense contractions resulting in 

 a corkscrew shortening of the branches and fibers occurred 

 subsequent to the fixation of the cells. It was not noticed in 

 any of the free cells even when two of them anastomosed before 

 becoming attached, and all the conditions appeared as favorable 

 as in the veliger larvae. After attachment such a shortening 

 is very noticeable, especially in the apical muscle and the fine 

 radial muscles of the side lappets. 



In an examination of these larvae, therefore, it is found that : 



1. The mesenchyme cells are at first nearly spherical and 

 are free-floating. In this condition they consist simply of a 

 nucleus and cytoplasm; they may put out amoeboid processes, 

 but they do not show any contractile movements. 



2. They soon grow larger, become granular, and develop 

 fibrous branches which hinder their free motion, and finally 

 they become fixed in position and function as muscles. 



