BLOOD-VASCULAR SYSTEM OF THE LORICATI 39 
however, it contains only venous blood. The heart proper con- 
sists of 2 chambers: a dorsal one, the auricle, and a smaller 
ventral one, the ventricle. The entering blood comes into the 
auricle posteriorly, from the thin-walled sinus venosus, from 
whence it is forced ventrad into the ventricle and then out an- 
teriorly into the elastic bulbus arteriosus. 
Stnus Venosus (Pls. 1, II, and VI, figs. 1, 12 and 39; S. Ven. 
and Sin. Ven.).—When inflated the dimensions of this thin- 
walled sinus are about equal. In a 4o lb. Ophiodon this cham- 
ber measured 38 mm.'in length from the entrance of one 
precaval vein to the other, 32 mm in breadth from the sinu- 
auricular valves to the entrance of the hepatic veins, and 28 
mm. in height at the center. The large sinus-like vessels 
emptying into this sinus have their inner edges reflected inward 
in the form of flaps, which tend partly to close the openings in 
case of a reverse current. In a like manner the walls of a sinus 
venosus, after having united with the outer connective tissue 
layer of the auricle, are reflected inward to form the s¢mz-auric- 
ular valves (Pl. VI, fig. 39; S.A.V.). Some fishes are said to 
have a dorsal and a ventral flap, but in Ophzodon they have 
become fused, forming a continuous circular flap, which de- 
creases the size of the sinu-auricular opening by at least one- 
half. 
Auricle (Text-figs. 1 ead; Pls. I, Il, and VI, figs. 1, 12, 39 
and 40; Aur.).—This triangular, saddle-shaped reservoir, con- 
vex above and concave below, is much larger than the ventricle, 
when inflated. It extends over three-fourths of the ventricle ; its 
anterior apex extends cephalad over the bulbus arteriosus some 
little distance, and posteriorly the auricle ends in 2 lateral horns. 
In this specimen the auricle measured 34 mm. in length, from 
its apex to the sinu-auricular valves, and if the posterior horns 
were included, the length would have been increased by at 
least 10 mm. The greatest width is in the neighborhood of the 
- posterior horns, where it is 40 mm., and the greatest height 
amounted to something like 22 mm. The walls of this cham- 
ber consist of 2 layers, an outer coat of connective tissue 
1 All measurements pertaining to the heart were taken from a 4o lb. Ophi- 
odon’s heart, which had previously been injected with a gelatin mass and hard- 
ened in formalin. ‘ 
