304 TOILERS IN THE SEA. 



with some of the cells (fig. 60). They are classified by 

 Krohn under three kinds. Those which constitute 

 the proper " bird's-head process," those which re- 

 semble pincers, and those which partake of the form 

 of bristles or hairs. Van Beneden traced their 

 growth and development, without obtaining any clue 

 to their use or purpose. Dr. Reid describes the 

 bristles in a common British species thus : " At the 

 anterior part of the outer side of each cell (in Cellu- 

 laria scruposd] and immediately in front of the tooth- 

 like process there attached, are two pretty long 

 spines, and a rounded process, which tapers slightly 

 from its fixed to its free extremity. This rounded 

 process is open at the top, and is hollow in dead 

 specimens, but when alive it is full of a contractile 

 substance. In this contractile substance the end of 

 a hair-like curved filament, about the length of the 

 cell, is immersed. This hair-like filament is moved 

 about, by the contractile substance attached to it, 

 generally in jerks, after intervals of repose, and in 

 its movements sweeps the anterior and posterior 

 surfaces of the cell to which it is fixed. These 

 movements continue for a considerable time after 

 the animal inhabiting the cell has been dead. A 

 hollow rounded process, with a hair-like curved and 

 moveable filament projecting from it, is also fixed 

 upon the corresponding part of each cell. These 

 moveable hair-like filaments are analogous to the 



