YERTEBRATA ACRANIA. 151 



ectoderm on each side, and constitutes a neural (medullary) plate, 

 which becomes depressed in the centre so as to originate a neural 

 (medullary) groove bounded by neural (medullary) folds. These 

 folds gradually approach each other, and ultimately unite, so that 

 the trough-shaped neural plate, now V-shaped in transverse section, 

 is completely covered. It ultimately folds up into a tube, which 

 is converted into the nerve-cord by thickening of its walls. A 

 small opening, however, is left in front, and this neuropore is appa- 

 rently converted into the olfactory pit of the adult. Posteriorly 

 the union of the neural folds involves roofing over the blastopore 

 with consequent formation of a short neurenteric canal by which 

 the cavity of the nerve-tube communicates with the digestive 

 cavity, which now no longer opens directly to the exterior. 



The embryonic development is brought to a close by the 

 appearance of the larval mouth, anus, and first gill-slit. 



The larval mouth ( = velar opening of adult) appears as a small 

 round ciliated opening on the left side of the head, in the region 

 of the first myomere. The anus develops soon afterwards. The 

 first gill-slit is formed on the right-hand side, close to the median 

 line, as a ciliated rounded aperture leading out of the pharyngeal 

 region. 



II. LARVAL DEVELOPMENT. The appearance of the larva at 

 the beginning of this period may be gathered from Fig. 42, F. 

 The points of most importance in the further history are connected 

 with the development of the adult mouth and buccal cavity, gill- 

 slits, and atrial cavity. 



Adult Mouth and Buccal Cavity. The larval mouth becomes 

 wide and oval, and two folds grow out, one above it, the other 

 below it, which respectively become the left and right boundary 

 walls of the buccal cavity. Later on the mouth shifts round to 

 a median ventral position, while the two folds become at the same 

 time more prominent, fusing together so as to enclose a cavity, 

 the buccal cavity, outside the larval mouth, and provided with 

 an oval opening, the adult mouth, from the margins of which 

 buccal cirri grow out. 



Gill-Slits. It is convenient to classify these, according to the 

 order of their appearance, as primary, secondary, and tertiary. 



(1) Primary Slits. The first of these, already mentioned, is 

 succeeded by 13 others, developed on the right side of the mid- 

 ventral line, and passing well up the right side of the body. 



