402 WILLIAM NICOLL. 
singly, one fibre being situated between the ends of adjacent 
radial fibres. Their diameter is about ‘O006—0013 mm., but 
the fibres of the external layer are somewhat stouter than 
those of the internal layer. ‘'I'wo distinct varieties of cells 
are met with in the sucker. The most numerous are small, 
rounded cells, which stain uniformly purple with heemalum- 
eosin, and their nuclei are differentiated only by their darker 
colour. They are situated for the most part closer to the 
external membrane than to the internal membrane. Several 
filaments are given off by each cell. The other variety of cell 
is distinguished by being much larger and by different staining 
reaction. The cell body is not well defined, but the nucleus 
is large, conspicuous and round or oval in shape. It stains 
light pink, with a clear zone externally and a more granular 
zone internally. Within it is a small round nucleolus, 
staining bright red. ‘hese are the myoblasts, and they lie 
in the spaces between the groups of muscle-fibres and usually 
midway between the outer and inner membranes of the 
sucker. 
The circum-oral papille are, as Looss has already pointed 
out, outgrowths of the muscle substance of the sucker. ‘They 
are six in number, two being distinctly ventral and four 
dorsal, and their bases are all situated on the same transverse 
plane. The ventral papillae are closely opposed to the 
inargin of the aperture of the sucker, and their bases, as 
above mentioned, occasionally give rise to two prominences 
projecting into the aperture; the other papille are situated 
equidistant from each other and from the ventral papille. 
The shape is depressed, tongue-like; the size varies according 
to the state of contraction. In a transverse section of their 
base the measurements are *035—04 by *015—02 mm., while 
the length is about ‘05-04 mm. ‘The muscle-fibres of the 
papille: are apparently finer than those of the sucker. ‘They 
comprise both longitudinal and annular fibres. No myoblasts 
were observed in the substance of the papillz, but at the 
base of each, situated in the sucker, there is usually at least 
one myoblast apparently in connection with the papilla. A 
