904 REPOET OF COMMISSIONER OF FISH AND FISHERIES. [14] 



(sp). WLen the larva moves freely, the part of the body which is in 

 front of the mouth is thrust entirely out of the shell, and is partly 

 folded. The vibratile zone in front of the mouth is composed of a 

 double row of long cilia. If the surface of the velum (Fig. 17) is ob- 

 served from above, it will be seen that these cilia grow on two rows of 

 cells, placed close by the side of each other and of almost rectangular 

 shape. From each of these cells there arise two vibratile cilia, whose 

 course msy be followed into the cellular protoplasm in the colored parts. 

 I have not been able to discover a vibratile zone in the posterior part, 

 although the portion of the head situated below the vibratile zone of 

 that part of the body which lies in front of the mouth is provided with 

 short vibratile cilia. The greater part of the surface of the velum is 

 at this period covered with a single layer of cells, exceedingly small, 

 so that they can be distinguished only by the i^lace occupied by their 

 stained nuclei. Only in the middle an enlargement may be noticed, 

 having a considerable depression toward the interior and composed of 

 several layers of ectodermic cells (Figs. 16 and 17, I-})). This is the 

 cephalic plate of which we have already made mention, whence orig- 

 inates the superior esophagean ganglion. Its surface seems to be di- 

 vided into two parts by a transverse furrow. I have not been able to 

 observe the peripheric nerves, issuing from the cephalic plate, which 

 Hatschek has observed in the larva of the Teredo. This enlargement of 

 the ectoderm seems to have been observed in the larva of the oyster by 

 Davaiue and De Lacaze-Duthiers; but both of them at first took it for 

 the mouth opening — an error which was afterwards recognized as such 

 by the last-mentioned of these two observers. 



The intestinal canal is also strongly developed, simultaneously with 

 the other parts of the body. The esophagus, covered with a brownish 

 pigment, has become elongated, and in its upper part has become 

 widened into a funnel. The gastric cavity, greatly enlarged, is by an 

 annular enlargement divided into an upper and lower part; in the 

 upper part there has been formed, to the right and to the left, a large 

 round pouch {I), the beginning of the liver, while the intestine begins 

 near the narrow part of the annular enlargement, and is folded in a loop 

 on the left side of the body, before opening into the mantle cavity. The 

 entire inner surface of the intestinal canal is lined with a vibratile epi- 

 thelium, except perhaps the pouches of the liver, the inner portion of 

 which is diflQcult to observe, owing to the great quantity of black pig- 

 ment.-^ 



The larva represented in Fig. 10 is the most advanced phase of free 

 larvae which we have observed ; this had been taken from the mantle 

 cavity of the mother oyster, or had been ejected by it when placed in 



2**These are the hepatic pouches, of dark color, which form the black spot by which 

 at an early stage in its development the oyster may be distinguished with the naked 

 eye. This spot is generally considered by fishermen as indicating the beginning of 

 the hinge. 



