500 MR. F. E. BEDDARD ON THE 
to each of the two smaller lateral ones, lies beneath the central larger tube. 
Each of these tubes (see Plate XX VII. figs. 3, 4, 5) is provided with a special 
fibrous sheath, outside which is another thicker fibrous sheath; these outer 
coats, however, are more or less continuous with each other, and with the 
septa dividing up the interior of the medulla, and perhaps ought not to be 
regarded as forming another special sheath to each of the tubular fibres. 
This description is more in accord with that of CLAPAREDE* for the common 
earthworm, than with the description of Pontodrilus by PErriER,t who denies 
the presence of a special sheath to each of the tubular fibres. The nerve cord 
of Pleurocheta is surrounded by a thick membrane, which has the appearance 
of elastic tissue ; in this are imbedded muscular fibres, sometimes singly and 
sometimes two or three together (fig. 3). The structure of the medulla itself 
varies according to the region from which the section is taken; fig. 4 is a 
section through one of the ganglia, and fig. 5 through the middle part of a 
commissure between two ganglia. The difference is at once apparent. There 
are no nerve cells in fig. 5. The nerve cells are developed on the under surface 
of the ganglia, and are found to extend some way along the commissures. 
Fig. 3 is a more highly magnified section through the middle of one of the 
ganglia. All the details given in the following description of the minute struc- 
ture of the nerve cord will be found represented in one or all of the figures 
already mentioned. 
Each ganglion is in reality composed of two fused ganglia, which is very 
clear on examining a section; the nerve cells are arranged in two lateral groups, 
and there are two circular areas separated off from the rest of the ganglion by 
septa of connective tissue, which are the interganglionic commissures uniting 
the different ganglia of the nerve cord with each other. In those parts which 
lie between the ganglia, the whole cord is made up of these commissural 
masses, there being no nerve cells present; these areas are occupied by a 
reticulum of connective tissue, in the meshes of which lie the nerve fibres, and 
a few small nerve cells differing altogether in size and appearance from the 
large nerve cells found in the ganglia. The rest of the ganglion is divided up 
by a finer meshwork of connective tissue, with stouter fibres here and there ; 
the nerve cells, which are pear-shaped, lie with their apices pointing towards the 
interior of the ganglion; the processes of these cells, which are for the most 
part unipolar, were generally traced into connection with the fibres constitut- 
ing the interganglionic connectives; each ganglion cell is provided with 
a large nucleus and nucleolus and lies in a space in the otherwise continuous 
meshwork. The fibres which make up the lateral branches given off in every 
* CLAPAREDE, loc. cit. 
t+ Perrier, Arch. de Zool. Exp., vol. ix. 
