142 LECTURE XII. 



The principal nervous productions of the brain of the leech are 

 what may be termed its crura, wliich diverge as they descend to 

 embrace the oesophagus, and are called the oesophageal chords ; 

 they then converge and reunite to join the large cordiform sub- 

 cesophageal ganglion (c). From this ganglion the muscles of the 

 three serrated jaws, as well as the principal muscles of the oral 

 sucker, derive their nervous influence. Those who have watched 

 the vigorous workings of this part in a hungry leech, beginning its 

 parasitic feast, will not be surprised at the great development of the 

 nervous centre of the suctorial and maxillary mechanism. Two 

 chords, in such close apposition as to seem a single nervous band, 

 are continued from the suboesophageal ganglion along the middle 

 of the under part of the abdomen, attached to the ventral in- 

 tegument, and inclosed, as it were, by the great ventral vein. 

 Twenty one equidistant rhomboidal ganglions are developed upon 

 these chords, which distribute their filaments to the adjoining 

 segments by two powerful diverging trunks on each side. The 

 segments indicated by the external circular indentations of the 

 integument are much more numerous than the ganglions. Dr. 

 Brandt has detected a simple nervous filament continued from the 

 oesophageal ganglion along the dorsal aspect of the alimentary 

 canal. This is an interesting structure, since it offers the first trace 

 of a distinct system of nerves, usually called the stomato-gastric in 

 Entomology, and to which our great sympathetic and nervus vagus 

 seem answerable. 



The structure of the abdominal ganglion in the leech has been il- 

 lustrated by the microscope and pencil of Ehrenberg : in the centre 

 of the ganglion are several clavate corpuscles, the enlarged end of 

 each being formed by a nucleated cell, the tapering extremity is con- 

 tinued into the diverging nervous chords : the clavate cells are 

 arranged in eight groups, two groups being continued into each of 

 the four diverging chords of the ganglion : one of these groups may be 

 supposed to be the recipient, the other the transmitter, of impressions. 



In the earth-worm the brain or supracesophageal ganglion consists 

 of two lateral lobes, which send off small nerves to the proboscis, and 

 the two large chords to the suboesophageal ganglion : some small 

 filaments are derived from the oesophageal collar. The two ventral 

 nervous trunks are more distinct from each other than iti the leech ; 

 but the ganglions are relatively smaller and more numerous, corre- 

 sponding in number with the segments of the body. Two pairs of 

 nerves are given off from each ganglion, and a third pair comes off 

 from the intermediate chords. The terminal or anal ganglion distri- 

 butes a plexus of nerves to that termination of the body. 



In the Nereis the abdominal ganglions are more distinctly bilobed 



