i6 



WILSON. 



[Vol. II. 



shape and position. There seems to be a special tendency to 

 their formation whenever the cell comes close to the wall of 

 the cavity, or when two cells approach each other. The 

 processes remain short and blunt, and in no instance were 

 they seen to reach either the wall or a neighboring cell. The 

 putting out of these processes was seen to be accompanied in 

 several instances by corresponding movements of the cell 

 contents very similar to those in an amoeba. 



Certain disturbances were also noticed in the liquid close to 

 the surface of the cell. At the time these were considered 

 analogous to slow ciliary motion, and they were probably 

 caused by "filose" action. 



It was impossible to determine with certainty whether the 

 amoeboid changes ceased and were followed by a period of 



rest before the cell became per- 

 manently branched; but this would 

 seem probable from analogy. 



At all events, permanent proc- 

 esses soon appear, the cells be- 

 come fastened in place, and after 

 subsequent development function 

 as muscles. 



During the development, after 

 the cells become branched, certain 

 activities appear which are of an 

 entirely different nature. 



A single cell, or more often a 

 pair of these mesenchyme cells, 

 mav be found in close proximity 



Fig. I. — Side view of veliger larva of Ter- "' 



gipes, showing position of the two con- to the internal Wall of the mantle 



tractile cells. Cam. luc. drawing, Leitz / n ^.i • i t\ 'j 



obiectiveNo.3,Eye.pieceNo.3 ^^ ^^^ (USUally thc right) SldC 



nearly in the center (Fig. i). 

 These two cells are in close proximity to each other, and 

 each consists of a body and radiating branches. The body is 

 composed of finely granular protoplasm, and contains a distinct 

 nucleus and several vacuoles. From it branches extend in 

 many different directions. There are usually several fine 

 branches reaching directly across from the body of one cell to 



