ON THE ANATOMY OV H1ST1U013DELLA HOMAlil. 323 



innervation of the jaw muscles ; they are dorsal and median 

 to the oesophageal commissures. I have been unable to make 

 out their connection with the muscles. 'J'hey take up methyl- 

 blue much more readily than do the other cells of the brain, 

 and retain it considerably longer. 



The ventral nerve-cord, like the brain, consists of a similar 

 central fibrous core, surrounded with nerve-cells. The two 

 halves of the cord are sepai-ated in the intersegmental 

 regions, joining up in the middle of the segments to form ii, 

 ganglion. From what can be judged from Haswell's draw- 

 ing^, in Stratiodrilus this separation is much less than in 

 Histriobdella. Unfortunatelv, most of the sections drawn 

 by Foettinger are taken through the middle of the segments, 

 and do not properly illustrate the extent to which the two 

 portions of the cord separate in the ititei'segmental regions. 

 The two halves of the cord are crescentic in transverse 

 section, the nerve-cells being imbedded on the ventral surface. 

 Where the coj-ds unite these cells are drawn out laterally to 

 form considerable masses on either side. 



The main ganglia, as already mentioned, correspond closely 

 with the five main segments into which the trunk is divided. 

 The first is situated in the anterior region of the first segment, 

 and is of considerable size. The second is somewhat smaller, 

 and is situated about the middle of the second segment. It 

 has fewer nerve-cells, and, like Stratiodrilus, it is placed 

 nearer the first than the third ganglion. The third is the 

 largest, taking up the greater part of the length of the cord 

 in the third segment, and having a great number of nerve- 

 cells. Between all the ganglia in the intersegmental regions 

 of the anterior segments the component parts of the cord 

 separate as already mentioned ; between the third and fourth 

 ganglia this is hardly perceptible, and from this point back- 

 wards to the tail region the two portions of the cord are in 

 close union, with the exception of a small area near the end. 



The fourth ganglion is the largest of all, and occupies the 

 middle of the generative segment. The fifth is in the middle 

 of the caudal segment. The position of these ganglia can be 



