496 W. JR. Coe — Aiiatoniy of Cerehratulus lacteus. 



this covering does not beconae single layered, as in the other vessels, 

 until much farther back. 



The Nephridial System. 



According to Burger (3) the Nephridial system in the Nemerteans 

 was first noticed in 1851 by Max Schultze (12) who found a second 

 pair of canals, which he designated as water-vessels, running parallel 

 with the lateral blood-vessels. No one seems to have again noticed 

 the nephridia until 1876, when Semper (13) found them in 3Ialaco- 

 bdella. The next year von Kennel (4) described in detail the system 

 in the same genus. More recently Hubrecht (8), Oudemans (9), and 

 Burger (3) have contributed more or less extensive articles devoted 

 almost entirely to the subject. In this species the nephridia (Plate 

 XIII, figs. 1, 5) occupy about the middle third of the oesophagal 

 region, and occur below and at the sides of the oesophagus. They 

 are not found above the oesophagus nor near the proboscis-sheath. 

 Leading to the exterior is a single pair of efferent ducts {iie). Each 

 duct is formed by the union of two longitudinal canals running in 

 opposite directions near the lateral nerve-cords, and parallel with the 

 longitudinal axis of the body. Of these the one in front of the 

 efferent duct is the longer. Each longitudinal canal divides dichoto- 

 mously into a large number of branches (u) which ramify among the 

 blood-vessels surrounding the oesophagus. Some of the smaller twigs 

 run for considerable distances parallel with, and close beside the 

 smaller branches of the blood-vessels. The excretory products con- 

 tained in the latter may readily pass through, or between, the few 

 cells separating the lumens of the two canals, be taken up b}" the 

 glandular epithelium lining the nephridial canals, and be swept by 

 the cilia to the outside of the body. 



The walls of the canals are made up of a thin layer of connective 

 tissue supporting the simple ciliated epithelium (Plate XIII, fig. 8, e) 

 lining the lumen. They are apparently destitute of muscular fibres. 

 The deep-stained, oval nuclei of the epithelial cells are placed closely 

 side by side. The cell-walls are indistinct in all the preparations. 

 The efferent ducts (fig. 8) differ in their structure but slightlj^ from 

 the other canals except that the former are provided with a much 

 thicker sheath of connective tissue (c). These ducts (Plate XI, fig. 

 3, ue) are often slightly enlarged just outside the circular muscular 

 layer and open by a small pore on the dorso-lateral surface of the 

 body. 



IJubrecht (7) states that he finds a direct communication between 



