106 Macueay Memoriat Vouume. 
He further points out the presence of the layer of diagonally disposed fibres 
previously overlooked. 
The muscular fibres of the sucker are a specially developed part of the muscular 
layers of the body-wall, with the addition of elements from the parenchyma muscle. 
Six sets of fibres are to be distinguished :—(1) Fibres which pass from the dorsal 
wall of the body to near the centre of the concavity of the sucker ; (2) oblique fibres 
which run through the substance of the outer part of the sucker from the dorsal to 
the ventral surface ; (3) fibres which run longitudinally from the ventral body-wall 
obliquely through the lateral parts of the stalk ; (4) radial fibres ; (5) circular fibres 
running round the margin ; (6) accessory fibres. 
Vi.—Inrecumentary GLAnDs. 
The very remarkably developed unicellular glands have received a fair amount 
of attention both from Weber and myself; but one or two points of some 
importance remain to be noticed.* Weber rightly remarks that I had understated 
the extent of the region of the body in which these unicellular glands are found. 
They are chiefly arranged in a broad band near the lateral borders of the body from 
the terminal sac of the exeretory system in front to the sucker at the posterior end. 
They are large cells—as much as ‘075 mm. in long diameter—mostly elongated in the 
dorsi-ventral direction. At its ventral end, as pointed out by Weber, each cell narrows 
to a process which acts as the duct. The cell has a large nucleus (-012 mm. in diameter 
in 7. fasctata). This is enclosed in a thick nuclear membrane and contains a single 
spherical nucleolus with a network of achromatin fibres. The protoplasm usually 
contains many of the rod-like bodies described below : sometimes it presents an open 
network of fine branching threads, for the most part radial in direction ; sometimes the 
network is close and dense. The rod-like bodies in the protoplasm of the cell are 
brought out most clearly in corrosive sublimate specimens stained with hematoxylin, 
which colours the rods much more strongly than the protoplasm. 
The secretions from these glands find three principal destinations, and, conse- 
quently, the glands themselves may be arranged in three groups. The glands of the 
first group—the tentacular glands—send their secretion forwards to the tentacles 
(Pl. xiv. fig, 1, 4. g.; Pl. xv. fig. 1, ~ g.). Those of the most posterior set—the 
acetabular —are connected with the sucker. The secretion from the cells of the third 
croup passes to the ventral surface of the body in the neighbourhood of the genital 
* As Weber points out, these unicellular glands cannot be regarded as modified cells of the parenchyma, but are 
almost certainly greatly produced epidermal cells, the body of which has become sunk deeply beneath the surface, with 
Which the cell only remains connected by the greatly elongated narrow process constituting the duct. 
