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. 



Sitius Venosus (Pis. I, II, and VI, figs, i, 12 and 39 ; S. Ven. 

 and Sin. Ven.). — When inflated the dimensions of this thin- 

 walled sinus are about equal. In a 40 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 sinii-aiiric- 

 ular valves (PL VI, fig. 39; S.A.V.). Some fishes are said to 

 have a dorsal and a ventral flap, but in Ophiodon 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, i and 2 ; Pis. I, II, and VI, figs, i, 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 



'AH measurements pertaining to the heart were taken from 340 Ih. Ophi- 

 odon's heart, which had previously been injected with a gelatin mass and hard- 

 ened in formalin. 



