76 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
The musculature and vascular systems are substantially the same as in the larger 
specimens. There is, however, an epidermal layer present (plate XXVII, fig. 6, a) 
which was absent from the larger specimens. This epidermis appears in transverse sec- 
tions as a border of short, curved, hair-like processes springing from a thin basement 
layer, which separates easily from the cuticle. The remaining layers are much like 
those of the larger specimens, except that the inner circular layer and the radial fibers 
are not yet differentiated. The layers named from the outside toward the center are : 
(1) the epidermal layer, (2) a thin, structureless cuticle, (3) a narrow layer of fine 
longitudinal fibers, (4) a dense nuclear layer, (5) a comparatively broad layer with 
scattered nuclei and a few calcareous bodies, and the vessels of the peripheral system, (6) 
a rather narrow but very persistent and strongly marked layer of longitudinal muscles. 
Within the layer of longitudinal muscles there is no further differentiafiou into layers. 
In it, however, a net-work of fine connective fibers, with numerous nuclei, a few calca- 
reous bodies,'the aquiferous vessels and marginal canals, are clearly revealed in the 
stained sections. In some of the sections a slight tendency to parallelism in the con- 
nective fibers immediately within the layer of longitudinal muscles indicated the be- 
ginning of the layer of circular muscles which occurs at this place in the larger forms. 
The marginal canals and the aquiferous vessels have essentially the same struct- 
ure as the same vessels in the larger specimens. 
The principal aquiferous vessel lies rather closer to the marginal canals than is 
the case in the larger specimens. As in the larger specimens, the marginal canals are 
characterized by their relatively large size, absence_ of proper walls, abundance of 
nuclei surrounding them, and a spongy, unstained interior tissue. Close to the central 
border of each of the marginal canals is the smaller aquiferous vessel (plate XXVIl, 
fig. 6, ff), with proper wall, open lumen, and in longitudinal section an irregular, crum- 
pled, and interrupted outline marking its tortuous course. Its wall is surrounded by 
nuclei. 
There are but few calcareous bodies in the central core. They are smaller and 
more uniformly elliptical in outline than those of the larger specimens. The largest 
measured .012 by .008 millimeter in their two optical diameters. 
The blastocyst has the following structure : Its investing wall consists of a num- 
ber of concentric layers, which in section appear like parallel fibers. These layers are 
rich in nuclei. In some of the sections in the vicinity of the embryo, these had 
become somewhat separated from each other, and as many as sixteen were counted. 
These layers appear to result from- the delamiuatiou of the outer portions of the 
parenchyma of the blastocyst. Nuclei exist in abundance in the external liortion of 
the mass of parenchyma on the side adjoining the embryo. 
For the most part the parenchyma appears uniformly granular and uon-nucleated. 
There is some evidence, however, of both contractile tissue and vascular structures. 
Several Nematods from these pyloric cmca, where they were eucapsuled in the 
serous membrane, a few also in cysts in the peritoneum, and one JEchinorhynchus were 
found in these trout. It has been thought best not to include descriptions of thorn in 
this paper. 
Washington, Pa., February 27, 1890. 
