. Comparative anatomy. Anatomy, Comparative. THE VASCULAR SYSTEM 353 Cyclostomes. The vascular system of cyclostomes shows little advance above that of amphioxus. An S-shaped heart with three cham- bers, sinus venosus, atriiim, and ventricle, is a novelty in this group. A conus with valves makes its appearance at the root of the truncus arterio- sus. The common cardinals drain into a thin-walled sinus venosus. In the adult the left common cardinal degenerates and both precardinals connect with the right common cardinal or precava as in some mammals. The two posterior cardinals unite into a com
. Comparative anatomy. Anatomy, Comparative. THE VASCULAR SYSTEM 353 Cyclostomes. The vascular system of cyclostomes shows little advance above that of amphioxus. An S-shaped heart with three cham- bers, sinus venosus, atriiim, and ventricle, is a novelty in this group. A conus with valves makes its appearance at the root of the truncus arterio- sus. The common cardinals drain into a thin-walled sinus venosus. In the adult the left common cardinal degenerates and both precardinals connect with the right common cardinal or precava as in some mammals. The two posterior cardinals unite into a common cardinal which passes to the left side of the body and unites with the sinus venosus. In the heart atrio-ventricular valves prevent a reverse flow of blood. With the development of a septum transversum the coelom becomes divided into SINUS VENOSUS. SEMILUNAR) VALVES Fig. 295.—A diagram of the primitive (fish) heart, as seen in a median longitudinal section. Anterior is to the right. The course of blood in the heart—^indicated by arrows—takes the form of a letter S. (Redrawn after Keith.) an anterior pericardial cavity and a posterior abdominal cavity. Red blood corpuscles make their appearance in this phylum and the bloodTs consequently red. A renal portal system is wanting in cyclostomes, the caudal vein draining directly into the postcardinal veins. Elasmobranchs. The blood vascular system of elasmobranchs differs little from that of cyclostomes and with slight changes may easily be derived from the latter. Associated with the appearance of paired fins the subclavian and iliac arteries and veins are present. The arteries are connected with the dorsal aorta. The subclavian vein is a branch of the precardinal, while the iliac vein drains into the lateral abdominal vein. The latter is believed to have been developed from the median ventral abdominal vein of the primitive chordates. Internal jugular veins paral- leling the precardinal veins are a novelty in this group.
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