The Lake and Brook La^npreys of New York 493 



so. Sensory organs, or nerve papillae. Compare plates V and VI. 



Fig. 52. (XA'A)- Frontal section, looking dorsad, of a larval lam- 

 prey, to show the velum and the course of the respired water. 



I, 2, 3. Branchiopores covered with valves extending from the ceph- 

 alic edge, d, b, b, b. Branchiae seen in section. 



A. Hood or upper lip. 



L Oral tentacle, v, v. Velum. The two independent velar folds 

 are shown m section. 



vl. Valve over the branchiopore. In this one it is open for the pas- 

 sage of the expiratory stream, which is indicated as passing between 

 the lamellae of two contiguous gills. 



The arrows show the stream entering the mouth through the straining 

 tentacles, then between the two halves of the velum into the branchial 

 chamber where it divides, part passing out between the gills of each 

 side, and through the corresponding branchiopores. 



Fig. 53, 54, 55. (X4/4). Three views of the valves at the opening of 

 the branchiopores that serve to direct the streams of water in inspiration 

 and in expiration in the adult. 



b, b. Branchiae seen in section. 



cl. Clavus or peg. A small, stiff, pointed body about 2 mm. high, 

 arising on the caudal margin, and opposite the middle of the branchi- 

 opore. In expiration the two valves meet at this point, as shown in fig- 

 ure 55. 



p. Fringe of papilla on the caudal margin of the branchiopore. 

 Their relation to the clavus is well shown in fig. 55. 



vl. Valves. There are two, one extending dorso-ventrad as 

 shown entire in fig. 55, and in section in fig. 53, 54. This one corre- 

 sponds with the single valve of the larva. 



The second, is a double valve within the branchiopore. In inspiration 

 the two parts move up against the cephalic side of the branchiopore and 

 are covered with the ectal or transverse valve and thus serve to guide 

 the water directly into the gill sac. In expiration, fig. 54, 55, they rest 

 against the caudal wall of the branchiopore, and, with the ectal trans- 

 verse valve, make a tube directed obliquely caudad, thus giving a cor- 

 responding direction to the expiratory streams of water. See also fig. 

 51, 52- 



