12 JEAN DAWSON. 



one another, the velar valves are relaxed and water may pass from 

 the water tube into the pharynx ; at the same time the velar jaws 

 are opened. When the rods are separated, the valves are 

 stretched so that they are able to close and the velar jaws are 

 also closed. 



C. Gills. In order to get a clear idea of the gills it is neces- 

 sary to call to mind their supporting skeleton. This consists of 

 nine irregular vertical bars on each side. The first is placed 

 almost immediately posterior to the styloid cartilage, the second 

 immediately in front of the first gill cleft and the remaining seven 

 are one just behind each of the seven gill clefts. These bars lie 

 close to rings of cartilage, which surround the gill clefts but the 

 bars are not continuous with the rings (Fig. 14). The vertical 

 bars are united by four longitudinal bars ; one is placed above 

 the gill clefts and one below them, while a third lies along the 

 side of the notochord ; the fourth lies close along the mid-vental 

 line and is connected with the corresponding bar on the opposite 

 side. The cartilaginous pericardial capsule is connected with 

 the ventral longitudinal cartilages at their caudal end and is very 

 elastic. This whole basket lies external to the gill sacs. While 

 it yields to every muscular contraction, yet it is strong enough to 

 lend firm attachment to muscles. 



Lying within this frame-work are found the gill sacs. Each 

 gill sac is a somewhat flattened ellipsoid. It is perforated at the 

 ends of its major axis by the external and internal branchial open- 

 ings. Its shortest axis is caudo-cranial so that we may con- 

 veniently distinguish in each sac a caudal and cranial surface or 

 wall, a lateral and medial end, and a dorsal and ventral border. 

 These sacs are not themselves attached to the supporting carti- 

 lage, but each lies within a muscular pouch to which it is con- 

 nected by muscular fibers, and this muscular pouch is in turn 

 attached to the cartilaginous rods of the visceral skeleton on 

 three sides, dorsal, ventral and lateral. 



The openings of the gill sacs into the water tube (internal 

 branchiopores) have slightly swollen lips. Those belonging to 

 each pair of sacs are in the same transverse plane. The opening 

 of each pair of gill sacs to the exterior (external branchipores) 

 lie also in the same transverse plane, but in a plane caudal to 



