IO4 JEAN DAWSON. 



branchiopore across the channel at the edge of the branchial sac, 

 and the ectal or transverse valve folds over the ental one. By 

 the expansion of the branchial apparatus, the entrance to the gill 

 sac has been rendered more direct and the inflowing stream flows 

 directly into the sac. In inspiration, the water flows through the 

 branchial lamellae, while around the edges, /. e., at the dorsal and 

 ventro-lateral edges of the gill sac there is formed a canal or 

 gutter by the shortening of the gill lamellae. The free ends of 

 the lamellae are also membraneous and curved and aid in making 

 a very complete and smooth canal. The ental valves at the 

 entrance to the branchiopore cross this canal and serve as a guide 

 to the inspiratory stream, not allowing the water to get into the 

 canal around the edges of the gill sac, but directing it into the 

 gill sac itself. In expiration, however, with the change in ob- 

 liquity and the constriction of the gill sac, the water passes 

 between the branchial lamellae into the canal and meeting the 

 ental valve rotates the two folds of the valves toward each other 

 and against the caudal wall of the branchiopore, thus removing 

 the obstruction in the canal and really extending it by means of 

 the arched valves. From this arrangement it is seen that two 

 distinct objects are attained, the water not only bathes the gills, 

 but passes between the lamellae, it is then concentrated in a canal 

 with smooth sides where the friction is at a minimum and in its 

 exit from the branchial sac in expiration the valves prevent the 

 used water from making a circle in the gills, and more important, 

 they form a very oblique channel which directs the expiring 

 stream caudad, thus insuring the animal against using the water 

 over and over. In inspiration, on the other hand, from the direc- 

 tion of the opening, the water enters at nearly a right angle to 

 the axis of the animal, and thus fresh or unrespired water is con- 

 stantly supplied to the gills." 



It has been shown above that these valves have another func- 

 tion than the one indicated by Gage. That they may also act to 

 direct the current of water within the gill sac as Gage believes, 

 does not seem to the writer necessarily to follow from the obser- 

 vations on record. The course that water may take within the 

 continuous space of the gill sac, during the inspiration and expira- 

 tion seems to be determinable only by direct observation or by 



