328 Annelides. 



and sucking-disk, so that the dissection of this part of the nervous 

 system is rendered rather more troublesome than that of any other of 

 the ganglionic masses. The brain gives off numerous branches to the 

 integument and sucking-disk, and ten very minute yet distinct nerves 

 to supply the eyes or ocelli, ten of which may be seen as little black 

 dots around the anterior and upper margin of the mouth, with a poc- 

 ket lens.* From the brain two cords are continued, which soon unite 

 and form the oesophageal ganglion, from which numerous branches 

 are given off to the muscles about the mouth, and likewise to those 

 which are attached to the three cartilaginous jaws. From this gan- 

 glion two cords extend backwards to the tail ; to the unassisted eye, 

 from their being so closely approximated, these two cords appear as 

 one, but by the microscope they will be rendered quite distinct. The 

 cords are enveloped in a strong sheath, containing much black pig- 

 ment, which prevents the white characteristic nervous structure from 

 being seen. At nearly equal distances in their course backwards, a 

 ganglionic enlargement takes place, from which a pair of nerves are 

 sent off on each side to supply the neighbouring parts. One of these 

 ganglionic enlargements is represented at fig. 4, magnified about 200 

 diameters. When the upper surface of the ganglion is in focus, you 

 have an appearance of striae, which are the fibres connecting the 

 nerves of one side with those of the other ; by altering the focus so 

 as to get the next layer into view, a number of small cells will be 

 made evident around the margin, each one having a nucleus, and 

 more towards the centre three or four larger cells may be seen, of an 

 elongated oval figure, also with nuclei, and with these the nervous 

 fibres appear to be connected. Ehrenberg, in his work on the struc- 

 ture of nerves, has represented a ganglion of the medicinal leech ; it. 

 appears to be precisely similar to that of the horse-leech, except in size. 

 Of these ganglia I cannot make out more than nineteen, exclusive of 

 the first or oesophageal ganglion, which is the largest : twenty-one are 

 generally given by authors as the number occurring in the medicinal 

 species. As a general rule, a ganglion occurs at every fifth ring of the 

 body ; but some of them are much nearer together than others, for in- 

 stance, the second ganglion from the oesophageal is much nearer to 

 the first than any of the others until we come to the last two or three, 

 which are placed near the anal outlet ; the third, fourth, fifth and 

 sixth are the widest apart ; they then get more closely approximated 



* See the upper extremity of fig. 1 ; the parts are reprcseiitctl on too small a scale 

 to admit of a (K'tailcd rcforciiec. 



