160 CHAS. B. WILSON. 



of movements in every possible direction and of great 

 delicacy. 



The larva is often seen advancing toward some object, or 

 toward the outer surface of the drop of water in which it is 

 swimming, and when still distant from it half its own 

 diameter it turns suddenly, sometimes even completely 

 around, and escapes coming in contact with it. Either, 

 therefore, the flagella are sensitive enough to recognise an 

 object when still some distance from it, or they are prolonged 

 beyond their visible tips, the prolongations being invisible 

 under the power used. 



In view of what we have already seen we must conclude 

 that the latter is far the more probable. 



The flagella also serve as a steering apparatus, like the 

 tuft of cilia at the apical pole of many Actiuian larvae, and 

 the apical plate in its earlier origin is sometimes traced back 

 to such a rudder-like tuft of cilia (cf. Korscheldt and 

 Heider) . 



Here in the pilidium we find both apical plate and apical 

 tuft, the latter retaining its original function of steering. 



Farther than this the flagella take no part in locomotor 

 movement, but are pre-eminently tactile, and constitute 

 practically the only sense-organ possessed by the larva. 



Their importance is shown by the comparatively complex 

 structure of the apical plate from which they arise, and by 

 the attachment to it of the largest and best developed 

 muscle in the whole pilidium. And yet they are exclusively 

 a larval organ, and take no part in the formation of the adult 

 worm, being thrown off with the epithelial covering. 



Sometimes there are two apical tufts instead of one as in 

 the figure here given. In such a case the plate is broadened 

 and thickened, but there is only a single apical muscle, 

 developed in the usual way. 



Pilidiums are sensitive to light, always congregating on 

 the side of the aquai'ium nearest the source of light, but they 

 show no response to ordinary sounds. 



Food. — The pilidium swims at the surface of the water 



