[Quahog Eggs]: Fig.15.--Ciliated gastrula, ten hours after fecundation. The embryo Can now swim through the water by means of hairlike cilia. The larger cells have become invaginated. Fig.16.--Trochosphere stage, twelve to fourteen hours after fecundation. The body has elongated and the cilia are now confined to the front end. The opening of the primitive mouth (pm) cn be seen on the lower side, while above is a slight indentation corresponding to the eginning of the shell gland (sg). Fig.17.--Formation of the shell, which arises at two symmetrical points of calcification, right and left of the median line, and gradually envelops the animal. Fig.18.--Early veliger swimmer with velum extended from the shell, about thirty-six hours after fecundation. aa, anterior adductor muscle, pa, posterior adductor muscle, s, stomach, a, anus, mt, mouth, v, velum. Fig.19.--Veliger slightly older than shown in Fig.18. The intesting (i) has elongated, and the liver (l) is more prominent.
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