ANKULATA. 249 



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 heginning 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 appo- 

 sition as to seem a single nervous band, are continued from the sub- 

 cesophageal ganglion along the middle of the under part of the 

 abdomen, attached to the ventral integument, 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 f>owerful diverging trunks 

 on each side. The segments indicated by the external circular in- 

 dentations 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, as being an 

 early 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 Articulata was 

 first illustrated by the microscope and pencil of Ehrenberg as it 

 exists in the leech* ; 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 continued 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 ; a 

 few filaments pass lengthwise through the ganglion, and are in more 

 immediate communication with the cerebral one.f 



In the earth-worm the super-oesophageal ganglion consists of two 

 lateral lobes, which send oiF small nerves to the proboscis and the 

 two large chords to the sub-oesophageal ganglion ; some small fila- 

 ments are derived from the oesophageal collar. The two ventral 

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

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

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



* Po^endorTs Annalen, 1833, and CLXXXVI. p. 665, tabt vi. fig. 6. (1834.) 

 f The homologues of these are pointed out by Mr. Newport in the Lobster 

 (CXCn. 1834, p. 406> Dr. Carpenter, in CXXXVUL (1839), cites Dr. M. Hall 

 as having " suggested that the ganglionic portion of the chord ministers to the reflex 

 actions of the respective segments, whilst the white tract (continuons filaments in 

 connection with the brain) conveys the motor influence of the cephalic ganglia " 

 (p. 62). He adds, that it may also " convey sensory impressions to those 

 ganglia "(ib.). 



