ANATOMY AND HISTOLOGY OF PLEUROCH ATA MOSELEYIT. 499 
Nervous System. 
The nervous system of Pleurocheta consists of a pair of cerebral ganglia 
fused in the middle line, but still perfectly distinguishable, occupying the first 
segment of the body, which are connected with a ventral chain by a pair of 
commissures. From either end of the cerebral ganglia a bunch of nerve 
filaments is given off, running towards the anterior end of the body. The 
commissures uniting the cerebral ganglia with the ventral chain are swollen in 
the middle, where they give off a number of nerves, one set from the anterior 
surface and another from the posterior. This part of the nervous system is 
represented slightly magnified on Plate SEXO fig. 1; the posterior part of the 
commissure is seen in this figure to be separated here and there from the main 
mass, and would appear to represent the rudiment of a visceral nervous system 
so generally developed in the Oligochwta, and to resemble more closely the 
visceral nervous system of Urochwta, which consists of a second cesophageal 
collar, rather than that of the other types of Oligochwta. In fig. 2 we have the 
anterior part of the ventral chain, together with the cerebral ganglia, and one 
of the commissures uniting the former with the latter. From the first ventral 
ganglion the nerves are given off anteriorly, but from all the rest the nerves are 
given off in pairs at right angles to the axis of the cord; from each ganglion three 
nerves take their origin on each side, of which two become united immediately 
after leaving the ganglion ; there is in each segment another pair of nerves given 
off between each of the ganglia. After the 12th segment the ganglia diminish 
considerably in size. The first ventral ganglion is placed in the second segment 
of the body, and following this there is one to each of the other segments. On 
the upper surface of the cord is a hyaline band extending along its whole length; 
this appearance may be produced by the “giant nerve fibres” lying on the 
dorsal surface of the cord. These structures, which are very general throughout 
the Annelida, have received various names ; they are the “giant nerve fibres” 
of Leypie, the “ tubular fibres ” of CLAPAREDE, the “ neural canal” of M‘INTosH ; 
they have been compared to the notochord of the vertebrate, and also to the 
neural canal, but this latter hypothesis is not borne out by the description 
of the development of the medulla in Lumbricus by KovaLevsky and by 
KLEINENBERG. Quite recently SpenceL* has described a single tubular body 
with coagulated fluid contents in the nerve cord of Echiwrus Pallasii. In all 
annelids where these structures have been observed, they are seen to consist 
of three longitudinal tubes filled with a coagulated fluid, and provided each 
with a special fibrous sheath; this is the case, for instance, in Lumbricus and 
in Pontodrilus. In Pleurochwta these tubes are four in number, three of which 
are arranged on the ordinary plan, and the fourth, which is about equal in size 
* SpencEL, Zeitschrift fiir Wissenschaftliche Zoologie, 1880. 
