ON THE BRITISH ANNELIDA. 267 



this tube in the enclosed span and forming the oesophageal ring. From the 

 infracesophageal ganglion the abdominal cord proceeds backward along the 

 ventral median line. In some species this cord is double, in others single ; in 

 some it is nodulated at intervals, corresponding with the annuli of the body, 

 and by ganglia ; in others it seems to consist only of a continuous cord. When 

 the former conformation prevails, the ganglia always bear reference in size 

 and number to those of the segmental divisions of the body. Whatever may 

 be the degree to which these ganglia are multiplied, they constitute only 

 repetitions of one another. The cephalic ganglion, representing in situation 

 and importance the brain of the higher Articulata, is distinguished from the 

 rest. It originates nerves to the organs of sense.. It is gifted, like a brain, 

 with the distinctive power of directing, controlling and coordinating the 

 movements of the entire body, whilst the influence of each ganglion of the 

 trunk is confined only to its own segment. The longitudinal ganglionic cord 

 occupies a position from which at first sight it may be inferred that it is not 

 the honiologon of the spinal cord of vertebrated animals. The spinal cord 

 affects, in relation to the body, a dorsal situation, the ganglionic chain of the 

 Articulata a ventral. This difference of locality has been supposed to be at 

 variance with the doctrine of the equwalence of these several forms of nervous 

 system. From the history of the development of articulate animals, however, 

 it has been suggested that the whole body of these animals may be considered 

 as in an inverted position ; the part in which the segmentation is first distin- 

 guished in insects being the equivalent of the dorsal region of vertebrata, and 

 that over which the germinal membrane is the last to close in, being homo- 

 logous with the ventral region. Regarded under this aspect, the longitudinal 

 nervous tract of Articulata corresponds with the spinal cord of vertebrated 

 animals in position, as it will be afterwards seen that it does in function. We 

 are indebted to Dr. Carpenter for an exact description of the ultimate 

 structural characters of these ganglia: — "When the structure of the chain 

 of ganglia is more particularly examined, it is found to consist of two distinct 

 tracts ; one of which is composed of nerve-Jibres only, and passes backwards 

 from the cephalic ganglion, over the surface of all the ganglia of the trunk, 

 giving off branches to the nerves proceeding from them, while the other 

 includes the ganglia themselves. Hence, as in the MoUusca, every part of 

 the body has two sorts of nervous connexions ; one with the cephalic gan- 

 glion, and the other with the ganglion of its own segment. Impressions 

 made upon the afferent fibres, which proceed from any part of the body to 

 the cephalic ganglia, harmonize and direct the general movements of the 

 body, by means of the efferent nerves proceeding from them. For the 

 purely reflex operations, on the other hand, the ganglia of the ventral cord 

 are sufficient, each one ministering to the actions of its own segment, and to 

 a certain extent also to those of the other segments. It has been ascertained 

 by the careful dissections of Mr. Newport, to whom we owe all our most 

 accurate knowledge ot the structure of the nervous system in articulated 

 animals, that of the fibres constituting the roots by which the nerves are im- 

 planted in the ganglia, some pass into the vesicular matter of the ganglion, 

 and after coming into relation with its vesicular substance, pass out again on 

 the same side ; whilst a second set, after traversing the vesicular matter, pass 

 out by the trunks proceeding from the opposite side of the same ganglion ; 

 and a third set run along the portion of the cord which connects the ganglia 

 of different segments, and enter the nervous trunks th&t issue from them at 

 a distance of one or more ganglia above or below. Thus it appears, that an 

 impression conveyed by an afferent fibre of any ganglion, may excite a 

 motion in the muscles of the same side of its own segment ; or in those of 



