REPOET OF COMMISSIONER OF FISH AND FISHERIES. [ 158 ] 
from the sides of the mantle (v, vc'): one of them ( vc '), receiving a 
branch from the gill, runs from the anterior part backward; the other 
( v from the middle part forward; these unite into one trunk before 
reaching the vence cavce. The posterior vence-cavce (vc") arise mostly 
from the caudal fins, but receive branches from the postero lateral por¬ 
tions of the mantle; each one receives two large branches, one anterior 
and the other posterior, just at the point where it leaves the inner sur¬ 
face of the mantle. From this point they run forward, parallel with the 
two posterior arteries, and converge to the region of the heart, where 
they join the great sacculated venous vessels; along a considerable 
portion of their course they expand and become large, elongated, fusi¬ 
form organs (r"), probably renal in function, but much firmer, more 
definite in form, and finer in structure than the more anterior renal 
organs. 
The gills ( g ) are long, triquetral, acute; in section they are nearly tri¬ 
angular (PI. XXXII, fig. 3), with the free ventral sides convex, and the 
dorsal side flat or concave, except along the middle, where a thin me¬ 
dian membrane ( d ) arises from a central ridge and unites the gill to the 
inner surface of the mantle. The gills are composed of large numbers 
of thin, transverse branchial laminm (Fig. 3, a), which extend outward 
symmetrically, on each side, from the large median blood-vessels (bo, 
6r), each half of a lamina having a long ovate or elliptical outline. A 
somewhat firm central axis or column ( c ) gives support to the laminae 
and the large blood-vessels. The great afferent vessel (bv) starts from 
the branchial auricle and runs along the median dorsal side of the gill, 
on the inner edge of the axial column (c) ; another parallel venous trunk 
or sinus ( v) is seen near the dorsal edge of the column. Each branchial 
leaf receives from the afferent vessel (bv) a branch (b) which runs along 
the dorsal edge, giving off at regular intervals small transverse paral¬ 
lel branchlets, which in turn give off minute capillary vessels along their 
sides and fade out near the ventral border of the lamellfe. Parallel 
with these arise small capillary efferent vessels, which join larger trans¬ 
verse vessels between and parallel with the afferent ones; these in turn 
join the larger efferent vessel that runs along the ventral edge of the 
lamina, and these marginal vessels pour their contents into the large 
vessel (bo) which runs along the middle of the gill on the ventral side 
and carries the purified blood to the heart. 
The alimentary tract is represented in a nearly dorsal view in Plate 
XX, fig. 2. In this figure the pharynx is shown in longitudinal section 
in a side view. The buccal membrane (bm) ; the pharynx with its horny 
jaws (sm the superior, and ini the inferior mandibles); the odontophore, 
(od) armed with seven rows of recurved teeth on the radula; and the 
thin chitinous lining membrane, which bears numerous sharp, scattered, 
recurved teeth, both on the palate and in the throat, have already been 
described (pp. 134, 135). The oesophagus ( oc ) is a long, narrow, but dilat¬ 
able tube, having two oblong salivary glands (sg) attached to it just at the 
