33 



tlie posterior vesicle coutiniie to increase in size, and by 

 the eventual disappearance of the median connecting 

 portion, become right and left enteroccelic sacs. About 

 the fourth day the embryo assumes a still more elongated 

 form, and at its anterior end a tuft of long cilia appears. 

 This is borne by a thickened and slightly depressed area 

 of the ectoderm known as the neural plate, in the deeper 

 laj'ers of which the rudiments of the larval nervous system 

 appear. Close behind the neural plate, on the antero- 

 ventral face of the embryo, is a slight depression, the 

 adhesive pit, by means of which the' free swimming larva 

 eventually attaches itself. The right enteroccelic sac 

 invades the segmentation cavity and spreads dorsally 

 forwards and over the intestine. The left sac enlarges 

 in the opposite direction and surrounds the posterior 

 border of the intestine. The rudiment of the hydroccel 

 is now separated from the intestine, but remains for a 

 short time in open communication with a small outgrowth 

 from its anterior wall, the parietal sinus. The intestine 

 undergoes changes of shape and becomes a vesicle. The 

 rudiments of the skeletal system now make their appear- 

 ance in the form of five oral plates, five basals, from three 

 to five infrabasals, and about eleven segments of the stalk 

 (fig. 65, jt. sk.). Externally the embryo is encircled by 

 five ciliated bands (cI. v.), and there is a depression in 

 the ventral ectoderm, known as the vestibule (vs.), the 

 significance of which will appear later. At this stage the 

 embryo is hatched out and becomes a free-swimming 

 larva, Avith the tuft of long cilia directed forwards. 

 Between the third and fourth ciliated rings there is a 

 minute aperture, the primary water pore. During the 

 free swimming stage the larval nervous system attains its 

 highest development, and the adhesive pit enlarges and 

 becomes glandular. The vestibule becomes tubular, owing 



