794 THE AMERICAN NATURALIST. 
Notwithstanding the resistance of this valve, successful capillary 
injections of Berlin blue were obtained as well as good silver 
impregnations of the veins and arteries. The capillaries are 
enclosed between epithelial plates which form the upper and 
lower surfaces of the gill and are separated from one another 
by small columns of cells which bind together the gill surfaces. 
The vascular endothelium could be traced only to the beginning 
of the capillaries, but microtome sections leave little doubt that 
the endothelium also extends into the capillaries. 
The salivary sinus and the sinuses of the head (brachial, 
inner and outer buccal, optic, and cephalic), which are usually 
cited as examples of lacunz, must be considered to be enormous 
dilations of the veins. They are so large that it is impracti- 
cable to examine every portion of their walls, but silver impreg- 
nations of portions of several sinuses show that they are lined 
by an endothelium. All the sinuses have essentially the same - 
structure, so that the description of one sinus will convey a 
true conception of all. The outer buccal sinus surrounds the 
sides and dorsal surface of the oval pharynx as the pericardium 
invests the heart. The outer wall of the sinus is supported by 
the muscular ring at the base of the arms, and the inner wall is 
supported by the pharynx and by the muscular septum which 
separates the inner buccal sinus from the outer. The walls are 
formed by connective tissue and by the endothelium which it 
supports. Ten or fifteen small veins which arise in the peri- 
stomial membrane open into the ventral end of the sinus, 
and the brachial sinus opens into the dorsal end. The blood 
passes from the buccal sinus through the optic sinuses to the 
vena cava. Beside having definite walls, these sinuses, unlike 
lacunz, do not connect arteries and veins, but receive blood 
from the veins and return it to the veins. The wide dis- 
tribution of the capillary vessels, the presence of an endo- 
thelium around every blood-containing cavity, except possibly 
the hearts, and the absence of demonstrable lacune, all lead 
to the conclusion that the arterial and venous vessels of the 
squid are connected by capillaries so that the vascular system 
is closed. 
- BROWN UNIVERSITY. 
