The Lake mid Brook Latnpreys of New York 469 



also somewhat to the dorsal and ventral parts of the ectal 

 valve. The other or the cephalic edge of each valve is free. 



The action of the valves is as follows: In inspiration, the 

 two parts of the inner or ental valve turn away from each 

 other and are pressed toward the cephalic wall of the branchio- 

 pore 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 (Fig. 53). In expiration, the 

 water flows through the branchial lamellae, while around the 

 edges, i. e., at the dorso- 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 mem- 

 branous 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 in- 

 spiratory 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 (Fig. 53). In expiration, however, with the change 

 in obliquity and the constriction of the gill sac, the water 

 passes between the branchial lamellae into the canal and meet- 

 ing the ental valve rotates the two folds of the valve toward 

 each other and against the caudal wall of the branchiopore, 

 thus removing the obstruction in the canal and really extend- 

 ing it by means of the arched valves (PI. VIII, Fig. 54, 55). 

 From this arrangement it is seen that two distinct objects are 

 attained, the water not only bathes the gills but passes be- 

 tween 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 im- 

 portant, they form a very oblique channel which directs the 

 expiratory stream caudad, thus insuring the animal against 

 using the same water over and over. In inspiration, on the 

 other hand, from the direction of the opening, the water enters 

 at nearly a right angle to the axis of the animal, and thus 

 fresh or unrespired water is constantly supplied to the gills. 

 (See figures 51-55, PI. VIII). 



