78 THE ANATOMY OF AN AUSTRALIAN LAND PLANARIAN. 
Throughout their entire course, until they unite at the posterior end of the body, 
the two longitudinal nerve cords are connected by very numerous stout transverse 
commissures. So abundant are these commissures that they are visible in almost 
every transverse section (Fie. 4, ¢.c.) They are, however, best shown in horizontal 
sections (Fig. 13), in which they are seen frequently to anastamose with one another, 
instead of running straight across. 
From the outer sides of the nerve cords numerous fine branches are given off 
towards the nerve sheath or plexus. In the region where the testes occur there is 
usually one such branch visible between each two adjacent testes (Fig. 13). 
In transverse section the longitudinal nerve cords usually present the charac- 
teristic spongy or finely reticulate appearance, which has been so fully described and 
discussed by previous authors that I need say nothing more about it here. The 
figures given by Jijima (7) for his fresh-water Tricladians might almost have been 
drawn from my own sections of Geoplana. Numerous small nerve cells occur 
scattered in the substance of the cords. 
The Nerve Sheath or Plexus.—This consists of a close network of fine fibres, lying 
between the outer longitudinal layer of muscles and the special zone of rod-like bodies, 
and hence only a short distance beneath the epidermis (vide Figs. 4, 10, 18, 15, 7.s.). 
It extends completely round the body, and though most easily seen in tangential 
sections is also plainly visible im vertical ones. The meshes of the plexus are 
remarkable in that they are usually more or less rectangular in form. Histologically 
the fibres of the plexus agree with the main nerve cord. It is only rarely that nerve 
cells are found actually within the fibres, but they are very plentifully scattered in the 
spaces between them. Figure 14 illustrates all these pomts. The separation existing 
between the fibres and the nerve cells is somewhat remarkable. I have already noted 
the tendency of the nerve cells to become aggregated at the periphery and 
immediately outside of the ganglion (beneath it), and a similar state of things is 
perhaps recognisable in the longitudinal cords. This state of things perhaps indicates 
that the nerve cells and fibres originate separately and independently and come 
together only secondarily. 
In the fresh-water Tricladians, according to Jijima (7), the nerve plexus forms an 
irregular network, spread over the whole dorsal surface, but apparently he has not 
found it on the ventral surface. He also states that it hes immediately below the inner 
longitudinal fibres of the skin musculature. This, as I have already pointed out, is a 
very strong argument in favour of the view that the internal longitudinal muscle layer 
of the aquatic Tricladians is the homologue of the external longitudinal layer of the 
terrestrial forms. } 
