i] THE STRUCTURE OF CONVOLUTA 9 



Occasionally, however, on meeting with an obstacle it 

 rears its head-end, caterpillar-wise, relaxes the grip 

 of its flexed sides, readjusts them to the surface and 

 glides on with stealthy motion. 



Though we have called C. roscoffensis and C. 

 paradoxa simple worms, it is not to be inferred that 

 the structure of their bodies is really simple. Both 

 species possess a well-defined nervous system and 

 efficient sense-organs. At the front or "head" end 

 of the body, on the upper surface, a little way behind 

 the anterior end, lie two eyes right and left of the 

 median line (Frontispiece and Fig. 3). Though of the 

 simplest construction, each consisting of a minute 

 spot of orange pigment lying over nervous tissue, 

 the eyes are efficient for distinguishing light of 

 different intensities. Numerous orange-pigmented 

 glands, scattered over the surface of the body, function 

 probably as accessory eyes. Between the two eyes, 

 in the median line on the dorsal (upper) side of the 

 body of either species, lies the otocyst (Frontispiece 

 and Fig. 3, OT). It consists of a hollow sphere of 

 nervous tissue enclosing a space within which lies a 

 small lump of chalk. 



Like a pea in a thimble, the heavy, chalky mass, 

 or otolith, lies freely in the otocyst, and, if the position 

 of the animal change with respect to the line of 

 action of gravity, the vertical the otolith falls or 

 rolls on a new part of the otocyst- wall. Pressing on 



