222 COMPARATIVE ANATOMY CHAP. 



second, third, or fourth segment (in certain earthworms). The infra-cesophageal 

 ganglion is also by no means always placed in the second segment of the body ; 

 here and there it lies in the head segment, or in the first and second segment, or 

 it moves farther back farthest of all in Pleionc, where it lies in the fifth or seventh 

 segment. In the Hirudinea (Figs. 143 to 145) the infra-cesophageal ganglion fuses 

 with a few of the subsequent ganglia of the ventral chord, forming a considerable 

 infra-cesophageal ganglionic mass, in which, however, by the number and arrange- 

 ment of the diverging peripheral nerves, and by the arrangement of the ganglionic 

 cells and fibre commissures, its composition out of several ganglia can be more or 

 less distinctly recognised. The same is true of the large posterior (sucker) ganglion 

 (Fig. 145), which is composed of six fused ganglia. 



As far as the ventral chord is concerned, the presence of a single ganglion in each 

 segment is the rule ; but there are exceptions to this rule in the most varied groups. 

 Sometimes one or two accessory ganglia are added to the principal ganglion, 

 e.g. in the Serpulidce two ganglia are typically present in each segment. On the 

 other hand the ganglionic masses may become very indistinct or be altogether 

 wanting (Archiannelida, and isolated forma in the most varied groups). In 

 Sternaspis the ventral chord shows ganglionic swellings only in the most posterior 

 part of the body. The fusion of the two lateral halves of the ventral chord is often 

 limited to the ganglia, whilst the longitudinal commissures run as completely 

 separated strands. These again may lie so close to each other that they appear 

 externally to be one single strand. It may even in various forms come to a fusing 

 of the two commissures, so that the ventral chord then consists of a single strand, 

 which, e.g. in the Archiannelida, does not even show the segmental ganglionic 

 swellings, and then recalls in a striking manner the ventral strand of the Gordiidce. 

 From the ganglia of the ventral chord there arise on each side nerves, generally 2 or 3, 

 which run upwards in the body wall and innervate the musculature, the integument, 

 and the segmental sensory organs, where such are present. It has been shown in some 

 cases that this innervation takes place by. means of a sub-epithelial nerve plexus de- 

 veloped all over the body. Lateral nerves may also diverge from the longitudinal com- 

 missures between 2 consecutive ganglia. The same is the case in the most anterior 

 longitudinal commissures the cesophageal commissures. 



There is often (especially in Hirudinea) in the ventral chord a delicate median 

 strand of longitudinal fibres which is called the intermediate nerve. The ventral 

 chord is nearly always covered by a single or double sheath of connective tissue 

 (neurilemma sheath), in which longitudinal muscle fibres run, principally in Oligo- 

 chceta and Hirudinea. 



Certain tubes with a wide lumen and wall formed of neurilemma, which run 

 back in varying but always small numbers on the dorsal side of the ventral chord, 

 deserve special attention. These tubes, which are called neurochord strands or 

 giant nerve tubes, begin anteriorly in the cesophageal commissures or in the infra- 

 cesophageal ganglion, and stretch to the most posterior end of the ventral chord. 

 They were in all probability originally the neurilemma sheaths of giant nerve fibres, 

 arising from larger ganglionic cells which lay in the ganglia at different parts of the 

 ventral chord. Various observations make it probable that the nerve fibres in the 

 neurochord tubes in various Annulata degenerate, the neurochord tubes themselves 

 persisting as elastic organs of support, containing a watery fluid mixed with the 

 remains of the original nerve substance. Such degeneration is, however, denied by 

 some authorities. 



The typical position of the ventral chord, and generally of the whole central 

 nervous system of the Annulata, is in the body cavity on the inner side of the 

 musculature. As the brain very often passes into the hypodermis of the head with- 



