20 ON THE STRUCTURE OF TRACHYPTERUS ARCTICUS, 
membrane is thrown into delicate folds set in a reticular manner with longitudinal 
meshes. The linmg membrane is of similar character, but with coarser meshes, 
posterior to the intestinal valve. The anus is near the middle of the body, on a 
level with the sixty-fifth vertebra. 
Female Generative Organs.—Anteriorly the ovaries are two long bands closely 
applied medianly, uniting at about one-third from their anterior ends to form a 
single organ. The whole extends forwards into the body-cavity for two-thirds of 
its length. The left branch ovary is three-fourths of an inch longer than the right. 
The length of the whole to the end of the larger band is twelve and one-fourth 
inches, and to the shorter eleven and one-half inches. The whole structure is hollow, 
the small ova being budded off from its side. The ovary in other words is cystoarian. 
Kidneys.—A median ureter, wide near the external opening and gradually 
narrowing as we follow it upwards and forwards, leads to near the middle of the 
body-cavity, and passes there into the posterior end of the kidney. The kidney 
runs forwards from this point beneath the centra of the vertebral column, and 
protected by the transverse processes, and expands beneath the clavicles into the 
head kidney. The latter are separate and very distinct, and pass gradually, after 
uniting medianly, into the middle kidney. The latter is a single organ, though it 
gives evidence, by a median furrow above and below, of its paired nature. It is 
laterally compressed, and is broader at its ventral border, sloping imperceptibly 
spo , 
Fig. 8 
ih T ‘ ab 
Urinary Orean (x }). 
p.n., pronephros, or head kid- 
ney; 7.n., mesonephros, or middle 
kidney ; wr., ureter. 
upwards on each side to the dorsal edge. Near to the head kidney the lower border 
passes into a distinct face, while the upper border retains its sharp edge. Here 
then a section gives almost the figure of an equilateral triangle. The head kidney 
is directed upwards from the plane of the middle kidney—the upper edge passing 
on in a gentle curve, the lower going forwards and forming an angle. At the corner 
the lower aspect of each head kidney becomes reduced to a rounded border, but it 
expands again into a flat face, which is retained to the pointed extremity. The 
upper edge has the character already described for the rest of the kidney. There 
is no trace of a hind kidney. 
The Heart.—The pericardial sac lies just behind and below the gills, in front 
