THE HISTOLOGICAL STRUCTURE OF THE GILLS, ETC. 101 
the water tubes for respiration as the filaments themselves 
can carry on that function though only to a limited extent 
due to the thickness of the layer of epithelium covering them. 
The epithelium lining the water tubes is much thinner. 
In figure 3 these cells are shown (mes.) with their pro- 
cesses, interweaving with each other. The majority of the 
cells as is shown in the diagram lie along the connective tissue 
which forms the base for the epithelium, and send out their 
branches diagonally across the filament, but many of them 
are found in the center of the filament supported by their 
processes running to the sides. Frequently blood corpuscles 
are found in this reticulum (figure 3 b.c.) between the meshes 
formed by the strands. Blood vessels of large size are found 
running vertically through the interlamellar junctions in 
every gill (figures 1 and 2 b.v.). These blood vessels are 
‘composed of cells which are extremely flattened and united at 
their ends to form the tubes. The cells are curved in an are, 
the amount of curvature depending on the number of cells 
required to encircle the blood vessel. The mesoblastic cells are 
in turn attached to these cells by their processes or the meso- 
blast cell may lie against the blood vessels and send out its 
projections of cytoplasm to others of its kind in the reticulum. 
This structure insures the blood vessel remaining in the same 
position guyed as it were by strands of cytoplasm. The struc- 
ture of a blood vessel with its supports is shown in figure 6. 
At the base of the inner gill the filaments making up the 
lamellae instead of directly doubling back upon themselves as 
in the outer gill, are thrown upward into a fold which gives 
the furrow mentioned in the introduction. The furrow was 
examined in serial sections of entire gills and was found at 
the extreme anterior end of the gill although it was very shal- 
low at this point since the filaments are so short at the ante- 
rior end. It gradually deepened for a distance of about a cen- 
timeter along the edge of the gill until the maximum depth 
was reached which is continued along the gill for the remain- 
der of its length. This depth varies with the size of the 
animal and to a small extent in different species. The depth 
of the groove is from one to three millimeters. The maximum 
depth of three millimeters was only found in one very large 
