64. THE ANATOMY OF AN AUSTRALIAN LAND PLANARIAN. 
completely hidden within a large cavity, which I have called the peripharyngeal 
cavity, but it is sometimes protruded when the animal is preserved in spirit (Fig. 3). 
Figure 9 shows the pharynx lying within the peripharyngeal cavity in the retracted 
state ; it will be seen that it is attached to the dorso-anterior wall of the cavity, 
with its free end pointing backwards ; while the terminal, disk-like portion is closely 
folded together. Figure 8 shows the pharynx when about half protruded through the 
opening of the cavity; the terminal, disk-like portion is becoming expanded. Von 
Kennel (8) has aptly compared the protrusion of the pharynx in land Planarians to 
that of the human tongue. 
Although the peripharyngeal cavity itself cannot be considered as forming any 
portion of the alimentary canal, yet it will be convenient to say a few words about it 
in this place. Its general form, as seen in longitudinal section, may be gathered by 
reference to figures 8 and 9; it is wide in front, and narrows gradually posteriorly, so 
as to run out into a sharp angle, which appears in transverse section in figure 4. It 
communicates with the exterior by an aperture whose position has already been 
described. When the pharynx is completely retracted this aperture is very minute, 
though distinct, and transversely oval in form, with its margins flush with the general 
surface of the body. When, on the other hand, the pharynx is protruded the 
aperture is necessarily greatly distended in order to admit of its passage. I have 
already described, when speaking of the epidermis, the peculiar glandular tissue 
which lines the greater part of the peripharyngeal cavity. The cavity itself appears 
to me to be simply an invagination from the exterior, and its opening has, I believe, 
nothing to do with the true mouth of the animal, which is situated in the centre of 
the disk-like termination of the pharynx. A precisely similar condition obtains in 
such forms as Leptoplana and Cestoplana, and is admirably shown in the diagrammatic 
figures given by Lang in his work* on the Polycladia of the Gulf of Naples. (8) 
Histological Structure of the Pharynx.—tIn transverse or longitudinal sections the 
outermost layer of the pharynx wall is seen to be made up of a single layer of very 
small, richly ciliated, columnar cells. Von Kennel (3) regards this layer as ‘“‘ Hine feine 
homogene Schicht, die sich mit Picrocarmin gut farbt, und die sehr zahlreiche, aber 
kurze Cilien traigt; Kerne sind darin nicht zu erkennen, ebensowenig Zellgrenzen ; 
dennoch darf man annehmen, dass diese Lage aus vershmolzenen Plattenzellen 
gebildet ist.” This view must have been based upon unsatisfactory preparations ; 
certainly the cells are very minute, and by no means easy to make out satisfactorily. 
Near to the insertion of the pharynx these epidermic cells are completely obscured by 
numbers of closely packed blocks of mucous wedged in between them. These blocks 
themselves simulate a columnar epithelium, but their homogeneous, decnly stained 
* Unfortunately, I have been unable to obtain access to this valuable monograph during the progress of the present 
work, and have only been able to refer to copies of a few of the figures made before I left England. 
