MUSEUM OF COMPARATIVE ZOOLOGY. 201 



They have a ligamentous junction with the posterior border of the oblong ser- 

 rated plates which form the body of the jaw. 



In their simplest form there is a remote resemblance between these jaws 

 and the chaetinous teeth of the species of Branchiobdclla found parasitic on the 

 crayfish. 



Nectonema agilis Verr. 



Plate VIII. 



Almost every summer for a number of years we have captured at Newport a 

 worm of doubtful affinities, which seems to be the same as that described by 

 Prof. Verrill as Nectonema agilis.* The only description of Nectonema known 

 to me is found in the Proc. Nat. ]\Ius., 1879, p. 187. The present account will 

 be found to confirm in the main the excellent observations recorded there by 

 Prof. Verrill, and I hope add something to what little is at present known of 

 its highly interesting anatomy. As no figures have yet appeared of this worm, 

 a few are introduced in Plate VIII. to illustrate its general form. 



Three specimens of Nectonema were collected in 1883. These were found 

 free-swimming, and were taken in evening fishing 



The body is long, thread-like, round, with the lateral lines slightly flattened. 

 It has a light brown or straw color and nearly opaque walls. Nectonema moves 

 through the water with great rapidity, coiling and uncoiling itself with tireless 

 energy as it swims. The length of the body varies in the specimens studied, 

 from four to six inches. 



The external body walls are smooth and unsegmented. Generally opaque, 

 in some specimens internal organs can be seen through its sides. The two 

 extremities are for the most part the most transparent. 



From one extremity to the other on each side there extends a well-defined, 

 broad, lateral liand, which is laid out in s(piares marked in outline by blaclv 

 pigment. This superficial marking may define a deeper anatomical segmenta- 

 tion, or it may be confined to the surface of the body and the walls immediately 

 adjacent below the surface. Each square is separated from its neighbors by a 

 raised band. The dorsal f sides of the squares appear the most densely jiig- 

 mented. A granulated line of cells extends from one end to the other through 

 all the lateral squares which make up the lateral bands. 



In some specimens there is found on the lateral bands two rows of hairs 

 closely crowded together, so that they sometimes appear in bundles. These 

 hairs seem to be connected by a muscular web, and are generally covered with 

 mucus and foreign matter which has become attached to them. 



Although the two extremities closely resemble each other, there exists a 

 marked difference between them in minute anatomy. 



* My attention was called to his description by Prof. Verrill, on beincr asked to 

 identify one of my specimens. 



t Dorsal and vgntral lines of the body of this filiform worm are very difficult to 

 distinguish. 



