18 ON THE ANATOMY OF MEGASCOLIDES AUSTRALIS, 
the stomato-gastrie system. The walls of the pharynx are well supplied with nerve 
fibres, which run backwards from the commissures. (Plate 3, fig. 10, ne.) 
Where the two latter join the ventral cord ganglion cells again appear, and are 
developed along the whole length of the ventral cord. 
Along the cord the connective tissue is arranged in a very definite manner. 
There is first of all the external coat immediately beneath the ccelomic epithelium. 
On either side this passes off in the region of a septum into a flat mesentery supporting 
the cord (Fig. 19), and as two strands of tissue are given off, one running from 
the upper and the other from the Jower side of the cord, a somewhat triangular 
space is enclosed in which longitudinal muscle fibres are enclosed. Similar fibres 
are present in other worms, as figured by Benuam in Microcheta rappi.* Within 
the cord itself are definitely arranged strands of connective tissue, in addition 
to the still finer tissue penetrating in and amongst the nervous elements and serving 
for their support. The cord is clearly seen in sections to be composed of two lateral 
halves, and may be divided into (1) a central double division, consisting of two halves, 
lying closely side by side and separated from each other and surrounding structures 
by a thin, but distinct, layer of connective tissue. These two parts are each composed 
of a meshwork of nerve fibres, which, in very many cases, can be traced into connection 
with larger or smaller ganglion cells lymg externally. The fibres, many of which are 
seen to curve in towards the middle line, very frequently have slight varicose 
swellings; and amongst them, more especially towards the ventral surface, and near 
the middle line, are a certain number of small nucleated ganglion cells. (Fig. 19.) 
The central part of the nerve cord is noticeable by reason of its not staining 
nearly as deeply (with borax-carmine) as the outer layer, and from the very much 
looser way in which its fibres are arranged. (2) An outer part confined to the 
lateral and ventral aspects of the cord, composed of a much denser meshwork of fibres, 
containing ganglion cells of various sizes. These cells do not appear to be aggregated 
into definite ganglia, but are present in great abundance in the parts of the cord 
which lie between the points of origin of the three pairs of nerves which arise in each 
segment. The larger cells are distinctly pear-shaped (Fig. 17), and as a rule 
arranged so that their thin pointed end protrudes into the central part, and from it 
passes off a single fibre, curving round towards the centre of the cord. Some of the 
smaller cells appear to have a fibre passing off from each end, but the large ones never 
have more than the single one. Occasionally a cell may be so large as to stretch 
across the space between the outer and inner layer of connective tissue. This external 
layer takes stain more readily than the internal one, and containing numerous nerve 
cells, as well as a closer meshwork of fibres, appears in section much darker. 
Where the lateral nerves enter, the fibres run right through to the central part, 
with which alone they appear to be in connection. In this region, ganglion cells are 
* Q.J.M.S., Feb. 1886, Plate 16, bis., fig. 37. 
