THE ANATOMY OF AN AUSTRALIAN LAND PLANARIAN. 77 
to his observations is that there is also a nerve sheath surrounding the body and 
connected with the two longitudinal cords by branches of the latter, so that the 
essential agreement of the nervous system of the terrestrial Planarians with that of 
other Tricladians is complete. [Since this was written I learn from Jijima’s paper 
(7) on the fresh-water Tricladians that Lang has discovered a nerve sheath in 
Rhynchodemus. Unfortunately I have not been able to see Lang’s paper. | 
I will now describe the nervous system as I found it in Geoplana spencer. 
The Ganglion.—This is a bilobed organ, situate at the extreme anterior end of 
the body, just beneath the skin, and below the anterior termination of the alimentary 
canal. Its position is best seen in figures 10 and 11. The ganglion (Fig. 12) 
consists of a very finely granular, non-staining eround substance, in which great 
numbers of small nerve-cells are imbedded. These cells are scattered irrecularly 
through the ground substance, but are more numerous towards the periphery than in 
the centre of the ganglion. As von Kennel has already remarked, there is no special 
membrane around the ganglion, so that the latter is not sharply differentiated from 
the general connective tissue of the body, and in the case of some of the peripherally 
placed cells it is impossible to say precisely which belong to the ganglon and which 
to the connective tissue. Thus, on the ventral aspect of the ganglion there is a dense 
mass of cells which agree in appearance with the nerve cells, but seem to be outside 
the ganglion, although touching it. These cells may, nevertheless, be nerve cells, for 
I shall show presently that nerve cells are not confined to the ganglion and nerve cords. 
The nerve cells are very small, with relatively very large and distinct nuclei. It 
is difficult to distinguish the protoplasmic body of the cell, but sometimes delicate 
processes are distinctly visible which resemble on a small scale -the radiating 
processes of typical multipolar ganglion cells. 
The Longitudinal Nerve Cords.—Kach half of the ganglion is continuous 
posteriorly with one of the longitudinal nerve cords. The cords themselves are 
relatively thick, and are recognisable at once in all transverse sections taken behind the 
ganglion as two very faintly stamed or unstained oval areas lying beneath the 
alimentary canal, one on each side of the middle line. 
Von Kennel (8) affirms that the two nerve cords do not unite posteriorly :—‘‘ Dies 
sind dann die Anfinge der beiden Langsnerven, der bisher sogen. Balkenstrange, 
die dann parallel bis ans hintere Koérperende verlaufen, wo sie allmahlig dinner 
werden und, ohne sich miteinander zu verbinden, aufhoren.” My sections of Geoplana 
spencert show that in this species at any rate the contrary is the case. The two 
cords gradually approach one another posteriorly until in transverse sections only a 
single cord is visible, lying in the middle line beneath the alimentary canal. This 
cord then gradually thins out, breaking up into several fine branches. 
