344 Gil, MAN A. DREW. 



in time become more frequent and powerful. Finally the foot 

 IS thrust out of the shell, stretched ventrally and anteriorly^ 

 swelled up at the end, and held more or less rigid while the 

 cilia vibrate. After beiug held in this position for a few 

 seconds it is withdrawn, either to remain quiet for some time, 

 or to be immediately thrust out again. The earlier move- 

 ments are not very energetic, and as the side flaps have not 

 been formed, they are not like the movements of the adult. 



The first indication of the side flaps consists of a slight 

 longitudinal groove on the raid-line of the ventral surface of 

 the foot. On each side of this groove the foot grows to form 

 flaps (figs. 40 and 61) that lie side by side. 



Soon after the test is shed, a rounded knob develops on 

 the postero- ventral portion of the foot (fig. 86). This grows 

 quite rapidly, and forms the prominence that appears like a 

 heel (figs. 40 and 41). It soon stops its rapid growth, and 

 in the adult is comparatively small (figs. 48 and 49). In 

 this species it is comparatively tnuch larger in the adult 

 animal than in any of the other species that I have studied. 



The side flaps at first have smooth margins (fig. 40), but 

 papillas soon begin to be formed (fig. 41). The anterior 

 papillae are formed first, and new ones are added posteriorly 

 as the foot grows, until as many as thirteen pairs have been 

 formed (fig. 48). The number diifers with the size of the 

 individual. Sexually mature specimens may be found with 

 no more than eight pairs. The papillae are large, conical, 

 more or less pointed, and very sensitive to mechanical stimu- 

 lation. 



The movements of the foot oE this species when compared 

 with the movement of the foot of Yoldia are very deliberate, 

 but the foot is so large, and the muscles so powerful, that 

 burrowing is quite rapid. Individuals of this species seem 

 normally to live entirely covered by mud, in which they 

 wander around by slow thrusts and retractions of the foot. 

 Specimens do not seem to come to the surface of the mud to 

 remain for any considerable time, and it seems probable that 

 the greater part of the lives of individuals are passed beneath 



