THE ANATOMY OF AN AUSTRALIAN LAND PLANARIAN. 63 
This may be compared, in what I believe to be the correct manner, with the 
musculature of the terrestrial forms as follows :— 
Bipalium and Rhynchodemus. Geoplana spenceri. | 
Superficial’ Syston: { External circular & oblique fibres.| External oblique fibres. 
| External longitudinal fibres. : External longitudinal fibres. 
Internal longitudinal and 7 | Internal ie naa and more 
or less circular fibres. or less circular fibres. 
Deep Syaton | Transverse fibres (supra-neural). | ‘Transverse fibres (supra-neural). 
| Dorso-ventral fibres. | Dorso-yentral fibres. 
Thus it appears that in the fresh-water Tricladians the superficial muscular 
system is, usually at any rate, more highly developed, and contains more layers than 
in the terrestrial forms, and that the internal longitudinal layer of the fresh-water forms 
is the homologue of the external longitudinal layer of the terrestrial forms. On the other 
hand, the terrestrial forms have developed a much more extensive body, or deep 
muscular system than the fresh-water forms, and this is doubtless correlated with 
the changed habitat and the thickened form of the body as opposed to the thin 
flattened form of the latier. 
Perhaps the strongest argument in favour of the views here advanced with regard 
to the homologies of the muscle layers in the terrestrial and fresh-water Tricladians 
is the relative position of the nerve sheath. This, as I shall show later on, is 
situated in Geoplana just beneath the external longitudinal layer of muscles. In the 
fresh-water Tricladians, according to Jijima, the nerve plexus (or sheath) occurs 
immediately beneath the inner longitudinal fibres of the skin musculature. Now, if 
these two layers of muscles, called outer in one case and inner in the other, are really 
homologues, as I believe, then the position of the nerve sheath in the land and fresh- 
water Tricladians is the same, as might be expected on purely @ prior: grounds. 
D.—The Alimentary Canal.—The alimentary canal of Geoplana spenceri has the 
typical Tricladian form, consisting of a pharynx and three main branches, one of which 
runs anteriorly in the middle line while the other two run posteriorly, one on each side 
of the genital aperture. 
The Pharynx and Peripharyngeal Cavity.—The pharynx is large, and when pro- 
truded somewhat trumpet-shaped, consisting of a short cylindrical tube, terminating 
in a flattened, disk-like expansion at its free end. In the centre of the flattened disk 
is a small aperture, the mouth. During the life of the animal the pharynx is usually 
