The ventral vessel undergoes complicated changes and is represented in the adults of existing fishes by a series of important structures. Its post-anal portion comes with the atrophy of the post-anal gut to lie close under the caudal portion of the dorsal aorta and is known as the caudal vein. This assumes a secondary connexion with, and drains its blood into, the posterior cardinal veins (see below). In the region between cloaca and liver the ventral vessel becomes much branched or even reticular and--serving serving to convey the food-laden blood from the wall of the enteron to the capillary network of the liver--is known as the hepatic portal vein. The short section in front of the liver is known as the hepatic vein and this conveys the blood, which has been treated by the liver, into a section of the ventral vessel, which has become highly muscular and is rhythmically contractile. This enlarged muscular portion, in which the contractility--probably once common to the main vessels throughout their extent--has become concentrated, serves as a pump and is known as the heart. Finally the precardiac section of the ventral vessel--the ventral aorta--conveys the blood from heart to aortic arches. In addition to the vessels mentioned a large paired vein is developed in close relation to the renal organ which it serves to drain. This is the posterior cardinal. An anterior prolongation (anterior cardinal) serves to drain the blood from the head region. From the point of junction of anterior and posterior cardinal a large transverse vessel leads to the heart (_ductus Cuvieri_). [Illustration: From Boas, _Lehrbuch der Zoologie_, by permission of Gustav Fischer. Fig. 26.--Diagram to illustrate the condition of the Conus in an Elasmobranch (A), _Amia_ (B) and a typical Teleost (C). a, Atrium. v,v´, Valves. b.a, Bulbus aortae. v.a, Ventral aorta. c.a, Conus arteriosus. vt, Ventricle.] s.v, Sinus venosus.