272 Mr. G. A. Drew oti the 



liver, of which the left lobe^ II, is indicated, lias made its 

 appearance as evaginations of the mid-gut. The cerebral 

 ganglia^ eg, pedal ganglia^ pg, and visceral ganglia, vg, are 

 connected by commissures. The cerebral ganglia have been 

 carried some distance from their point of origin, but are still 

 connected with the exterior bj the unpaired space, r, which 

 has followed them in. The otocysts, ot, seem to be com- 

 pletely closed off, and each contains an otolith that stains 

 deeply with hematoxylin and a little later plainly shows 

 concentric structure. As the otocysts have never been open 

 to the exterior, development having taken place inside a 

 closed test, the otoliths cannot be foreign jjarticles. 



At about the age of one hundred and five hours, or a little 

 later, the embryo stops swimming, settles to the bottom, the 

 cilia shrivel, the test-cells break apart and go to pieces, and 

 the animal is left in its clear white shell free upon the bottom. 

 Beside the test, the casting includes the stalk that extends 

 from the test to the cerebral ganglia, the apical plate and its 

 connexion, and the ventral tube from the blastopore to the 

 position of the definitive mouth. The time occupied by 

 these changes is very brief, only a very few minutes at most. 



The foot, which at first is capable of only feeble move- 

 ments, grows rapidly, and is soon very active. It is never 

 moved slowly, but is thrust out with a jerk and withdrawn 

 quite as suddenly. Locomotion is chieHy effected by long 

 and powerful cilia, which are arranged along the sides of the 

 foot, and are very active whenever the foot is thrust out of 

 the shell. The animal is thus enabled to glide over the 

 surface on which it rests, but is unable to rise. 



The gill rudiments aj)pear in about one hundred and fifteen 

 hours as thickenings of the posterior portion of the mantle. 

 Each thickening becomes more pronounced, and is soon 

 divided by a constriction into two parts — one dorsal, the 

 other ventral. This constriction deepens, the ventral portion 

 broadens, is carried posteriorly, and in turn becomes divided 

 into two parts. In the meantime the alimentary canal has 

 lengthentd, the stomach has enlarged, the left liver-lobe has 

 become larger than the right, and the foot has acquired a 

 " sole," which roughly corresponds in shape and movements 

 w ith the " sole " of the adult. Locomotion is still materially 

 aided by the long and powerful cilia. Fig. 5 represents a 

 reconstruction of an embryo twelve and a half days old, as 

 seen from the right side, with the right shell-valve and the 

 mantle-lobe removed. The powerful cilia of the foot and 

 gill and the fine cilia that cover the surface of the body and 

 project into the lumen of the alimentary canal have not been 



