170 EMBRY0L0GICAL TYPES 



the left side, the endostyle assumes a midventral position, and 

 the secondary slits on the right side correspond more or less 

 symmetrically with the primaries on the left. The first 

 primary (left) gill-slit, and the club-shaped gland disappear, 

 and the number of slits on each side is regulated to eight by 

 the disappearance of the posterior primaries. After this stage, 

 more and more gill-slits are formed symmetrically on both 

 sides, and the segmental correspondence is lost. 



All the gill-slits except the anterior pair become sub- 

 divided into two by the downgrowth of the secondary or 

 tongue-bars. The perforation of the gill-slits naturally 

 obliterates the coelomic cavity at the place of perforation ; the 

 ccelomic cavity is therefore restricted to the primary bars 

 between the gill-slits, and to the dorsal coelomic canals above 

 and the subendostylar coelom below. The tongue-bars have 

 no coelomic cavity, being downgrowths across the openings 

 of the gill-slits. It is because of this difference in method of 

 formation between the primary bars and the tongue-bars, that 

 in the adult the former contain a portion of coelomic cavity 

 and the latter do not. 



During the rearrangement of the gill-slits, the mouth 

 moves round to the anterior end. Its aperture decreases in 

 size as its margin grows-in all round to form the velum. 



Folds of the skin give rise to the oral hood, in the roof of 

 which the preoral pit finds itself. The latter flattens out, and 

 its cells give rise to the wheel-organ, or ciliated organ of 

 Miiller. 



The Atrium. — The atrium arises as a pair of ventral longi- 

 tudinal folds, the metapleurs. These folds pass on each side 

 of the region of the gill-slits, which come to be situated between 

 them. From each fold, a median shelf or epipleur extends 

 and meets its fellow from the opposite side, thus enclosing a 

 part of the outside world as the cavity of the atrium. The 

 cavity is completely closed in front ; behind it remains in 

 communication with the exterior by the atriopore. The 

 atrium is lined throughout by ectoderm. 



The nephridia arise as little blind sacs eventually connect- 

 ing with the exterior, at the top of each gill-slit (before the 



