No. 430.] THE COMMON SQUID, LOLIGO PEALII. 789 
vena Cava opens into the apex of the V-shaped nephridial sinus 
which is surrounded by the nephridial chamber. The venous 
blood from the dorsal end of the mantle returns through the 
two postcavee which open into the ends of the arms of the 
nephridial sinus. This sinus opens at each side into a bran- 
chial heart which forces the blood through the gill to the 
systemic heart. The sinuses are not interposed between 
the veins and arteries but receive blood from the veins and 
return it to the veins. The aortz, the larger arteries, the 
branchial arteries and veins, the anterior vena cava, and the 
postcavee are all peristaltically contractile. The wave of con- 
traction passes slowly away from the heart along the arteries, 
and toward the heart along the veins. The peristalsis of the 
vessels continues long after the stoppage of the hearts and 
the apparent death of the animal. 
The Structure of the Arteries and Veins. — The systemic 
or arterial heart is composed of smooth muscle fibers. The 
external surface is firm, but the inner surface is formed by a 
latticework of interlaced fibers. The great irregularity of this 
surface interferes with the success of silver impregnations, 
and although some impregnations and microtome sections indi- 
cate that there is an endothelium, such a large number have 
yielded only negative results that it is probable that an endo- 
thelium is not present in the heart. The arterial blood which 
passes to the tissues of the heart from its lumen becomes 
venous and is carried to the nephridial sinus by two or three 
small veins. It is probable that these veins are separated from 
the lumen of the heart by capillaries. The vessels through 
which the blood passes are certainly as small as capillaries, 
but it was impossible to demonstrate an endothelium. 
The branchial hearts are quite different from the systemic 
heart in that the muscle fibers are cross striated and are sepa- 
rated by numerous clusters of apparently glandular polygonal 
cells, which possibly have a function in connection with the 
pericardial gland that is attached to the inner edge of the 
heart and receives a large blood supply from it. Silver impreg- 
nation shows that an endothelium is not present and that the 
muscular and glandular cells are in contact with the blood. 
