G. A. DEEW ON YOLDIA LIMATULA, SAY. 27 



mass of large cells, Figs. 35, 64 and 65, z, that are not arranged in 

 any definite manner. It was at first thought that this appearance 

 was due to poor preservation, an explanation that very naturally 

 suggested itself, but later the large cells were found in the living as 

 well as in the preserved specimens. Reformation of the digestive glands 

 is accompanied by the disappearance of these cells. 



The anterior adductor muscle soon becomes inclosed by the 

 forward growth of the body. For a few hours a space may remain 

 between it and the cerebral ganglia, but this soon disappears. 



The foot, which at first is capable of only feeble movements, 

 grows quite rapidly, and is soon very active. Locomotion is chiefly 

 effected by the means of long and powerful cilia arranged along the 

 sides of the foot, Figs. 35 and 36, which are very active whenever 

 the foot is thrust out of the shell. These enable the animal to glide 

 over the surface on which it lies, but they do not enable it to rise. For 

 some hours after the test is cast, the foot shows no indication of the 

 modification which is later generally known as the sole, Fig. 52. The 

 first indication of this modification is a groove which appears along 

 the middle line of the ventral surface of the foot, Fig. 53. This groove 

 deepens, apparently by the growth of the parts on each side, Fig. 54, 

 and gives rise to a "sole" resembling that of the adult, Figs. 3 and 9. 

 The movements of the foot now closely resemble those of the adult, but 

 locomotion on smooth surfaces is still chiefly by means of the powerful 

 cilia along, its sides. 



The formation of the " sole " is accompanied by the growth of the 

 postero-ventral portion of the foot into a more and more prominent 

 portion resembling a heel. The use of this outgrowth is not evident. 

 It remains in the adult as a small projection, just behind the opening 

 of the byssal gland ; an organ that makes its appearance about a 

 day after the test is cast, but does not seem to secrete threads. 



About the time that the test is cast away, a thickening makes its 

 appearance on the inside of each lobe of the mantle, near its posterior 

 border, Figs. 34 and 35 g. This thickening, the rudiment of the gill, 

 acquires cilia and grows anteriorly so as to project over the surface of 

 the mantle. A constriction appears near the middle of each gill, 

 Fig. 36, g, the lobes become more prominent, and it is thus divided 

 into almost equal parts. Beneath its epithelium, following the curve 

 of its anterior margin, is a chitinous rod, the two ends of which lie 

 near the ends of the lobes, Fig. 36. 



