216 BULLETIN OF THE BUREAU OF FISHERIES. 
This aorta I have just described is the morphological posterior aorta, though its 
course at first is anterior. The second aorta leaving the ventricle runs posteriorly 
in the visceral mass, which it supplies, and is the morphological anterior aorta. 
The venous system consists of three important parts. Blood from the viscere 
and anterior part of the body is gathered into a system of afferent branchial veins 
consisting anteriorly of a pair of large vessels (ba, fig. 27-29) which in the region 
of the visceral ganglion unite to form the single very large afferent branchial vein 
that runs between the fused gills (fig. 10, 30-82). Passing from this vein through 
the gill lamella, the aerated blood enters the large paired efferent branchial veins, 
from which it passes to the auricles. Blood from the posterior part of the body is 
gathered into an afferent renal vein (arv, fig. 31, 32), which runs forward and 
enters the perinephridial spaces at the posterior end of the kidneys. 
The description I have just given applies specially to X. gouldi and T. 
navalis. In T. dilatata, while the relations are somewhat different, the homol- 
ogies remain the same. In this species the principal part of the visceral mass has 
remained more anterior and the posterior part of the body is longer in proportion. 
In following the gills the heart has become much more elongated, and this elonga- 
tion has taken place principally in the aorta-like part of the ventricle which runs 
forward from the more thickened portion of the ventricle. In this species the 
pericardial cavity extends much farther forward than in X. gouldi, passing 
under and anterior to the posterior adductor muscle as a long canal to end under 
the esophagus. In this canal the ventricle runs to the anterior side of the posterior 
adductor and then dips into the visceral mass. Valves mark the anterior end of 
the very long ventricle, from which two vessels pass forward. The larger, after 
giving off branches in its course, bends around the adductor and divides into 
paired pallial arteries which supply the posterior part of the body. This is the 
posterior aorta. The other, the anterior aorta, also runs forward a short distance, 
but soon breaks up into arteries which supply the visceral mass. 
I have gone into details in describing the aortz, because the posterior aorta 
has been described as fused with the anterior in Teredo. This observation was 
first made by Grobben (1888) who described as aorta a part of the ventricle which is 
distinctly muscular and contractile. The parts which should have been described 
as aorte he has not figured at all. Menegaux has also maintained that the two 
aorte are fused (1889). Unfortunately neither of these authors names the species 
with which he worked, but their descriptions of other parts are faulty and indicate 
that there is little doubt that they have been in error in this regard also. 
ALIMENTARY CANAL. 
In adaptation to their burrowing mode of life, the alimentary canal of all the 
Pholadacea has become more highly specialized, perhaps, than in any other type 
of lamellibranch. This specialization is most accentuated in the ship-worms, 
apparently in association with the ingestion of the particles of the wood grated off 
in burrowing. 
Most of the parts of the alimentary canal of the adult are already present in 
the newly attached larva, though their relations to each other and their relative 
