THE CEPHALOCORDA 197 



afterwards shifted to the right. The peculiar organ shown 

 in front of the first gill-slit in fig. 50, B, is a transient em- 

 bryonic organ formed from the floor of, the pharynx, and 

 known as the club-shaped gland. The fourteen or fifteen gill- 

 slits first formed, and shifted to the right side of the body, 

 are known as the primary gill-slits. Above them on the 

 right side six secondary gill-slits are formed, and as the 

 latter increase in size they push the primary gill-slits first to 

 the ventral side, and ultimately to the left side of the pharynx. 

 The first and some of the more posterior of the primary gill- 

 slits close up and disappear ; two additional secondary slits 

 are formed, and so a stage is reached in which there are eight 

 pairs of gill-slits, more or less symmetrically disposed, on the 

 right and left sides of the body. While the gill-slits are being 

 adjusted to their final positions, the mouth also is shifted from 

 the left side to the anterior end of the pharynx ; thus to the 

 position which it occupies in the adult. It should be observed 

 that the gill-slits are simple oval apertures at first, but subse- 

 quently become horseshoe-shaped, and finally divided into two 

 by the downgrowth of the tongue-bars from their dorsal margins. 



For a detailed account of these remarkable phenomena and 

 the subsequent changes undergone by the larva, the reader 

 should consult Dr Willey's Memoir. We must confine our- 

 selves in this place to a consideration of the formation of the 

 atrial cavity and the fate of the mesoblastic somites. 



The atrial folds are formed in a larva with six or seven 

 primary gill-slits, as two low ridges or longitudinal thickenings 

 of the integument on the ventral side of the body. Anteriorly, 

 in correspondence with the asymmetry of the primary gill-slits, 

 the ridges diverge to the right. These ridges enlarge, and 

 become well-marked folds hanging down from the body-wall. 

 Then, from the inner face of each fold a solid ridge, the 

 epipleural fold, grows in towards the middle line and meets 

 and coalesces with its fellow of the opposite side. In this 

 way a tube lined by epiblast is formed enclosing the gill-slits. 

 As the epipleural folds do not coalesce behind, the tube opens 

 to the exterior by an aperture, the atriopore. Though small 

 at first, the atrial tube soon grows in size and extends itself 

 by dorsal upgrowths right and left of the pharynx, thus re- 

 ducing the perivisceral ccelom to very small dimensions. The 

 metapleural folds, as the lower edges of the atrial folds are 



