ON THE STRUCTURE OF VERMIOULUS PILOSUS. 257 



The alimentary canal is simple, and quite similar to that of 

 Tubifex. A short way behind the mouth (fig. 2, m.) is a 

 muscular pharynx in the 2nd segment (fig. Q, ph.), followed by 

 a slender oesophagus stretching through the 3rd and 4th seg- 

 ments (fig. 6, oes.), and from the 5th segment backwards a 

 straight intestine covered with brown chloragogen cells (fig. 6, 

 int.). The anus is terminal. The pharynx is provided with 

 irregular masses of glandular cells. From the pharynx back- 

 wards the alimentary canal is lined by ciliated columnar 

 epithelium, outside which is a thin layer of circular and longi- 

 tudinal muscles. In the wall of the intestine is a system of 

 blood-capillaries forming a plexus similar to that described in 

 the Enchytraeids and other Tubificids. 



The cerebral ganglia form a bilobed brain, deeply cleft in 

 front and situated in the 1st segment (fig. 6, br.). The struc- 

 ture of the ventral nerve-cord is somewhat exceptional. On 

 examining a transverse section taken between the ganglia 

 (fig. 16) J it is seen to consist of an outer covering of ccelomic 

 epithelium (fig. 16, c. ep.) enclosing the nerve-cord proper and 

 three strands of muscular fibres, of which the middle one 

 (fig. 16, m. n. c. m.) is the most important. The nerve-cord 

 itself is formed of two circular strands of ordinary nerve-fibres, 

 on the dorsal surface of which run three neurochordal fibres. 

 Owing to their delicate structure, these enlarged fibres are so 

 difficult to see in specimens preserved in corrosive and stained 

 with borax-carmine that at first I thought they were altogether 

 absent. They are, however, plainly visible in sections of 

 specimens preserved in Lindsay- Johnson fluid and stained with 

 paracarmine (fig. 15). A transverse section taken through a 

 ganglion (fig. 15) shows that the ganglionic cells extend on the 

 under surface of the cord, and rise up at the sides, actually 

 overlapping the muscular strands (fig. 15, g. c). In a section 

 taken at the point where the nerve-cord is passing through 

 the septum (fig. 14), we see that muscle-fibres pass from the 

 septum underneath the median muscular strand overlying the 

 cord (fig. 14, m. s.), while the lateral strands communicate 

 with those which descend on either side to the body-wall. 



