32 CLASS XIV. 



unites with that of the opposite side, and thus conducts the blood 

 to the auricle of the heart. What, therefore, is thus seen in the 

 embryonal state of other vertebrate animals remains in fishes as the 

 permanent form. There are properly no posterior cavae, which in 

 fishes are usually so named, but venous stems which correspond to 

 the venae cardinales of the embryo, and of which in osseous fishes 

 that of the right side is usually much the most developed, so that 

 a single posterior cava alone is ascribed to them. The venous stem, 

 or the veins that come from the liver (vena hepatica or vence hepaticce), 

 join the common venous sinus into which the two ductus Cuvieri 

 open. The hepatic veins alone are those that can be compared 

 with the posterior cava 1 . 



In Anguilla and Murcenophis the veins of the caudal fin unite 

 to form a pulsating venous heart on each side of the last caudal 

 vertebra 2 . In Myxine the portal vein is distended into a large sac, 

 which contracts and expands alternately 3 . In fishes, as in other 

 classes of animals, arterial and venous plexuses (retia mirabilia 4 ) 

 occur, in which the stem is suddenly lost as it were, and of which 

 the vessels at first lie side by side without dividing into branches, 

 but afterwards either pass into capillaries or unite to form one 

 or more larger trunks. They occur in the vessels of the viscera 

 in Thynnus and some sharks, also in the swimming bladder of 



1 H. RATHKE, Ueber den JBau und die Entwickelung des Venensystems der Wirbel- 

 thiere; dritter Berickt ilber das Naturwissensch. Seminar zu Konigsberg. 1838, 4to. 



2 According to the discovery of MARSHALL HALL in the eel; according to MUELLER, 

 in Murcenophis also. At the same part many osseous fishes have a sinus, which belongs 

 to the lymphatic system of vessels. See HYRTL in MUELLER'S Archiv, 1843. s. 224 

 240, with fig. 



3 MUELLER'S Archiv, 1842, s. 477. 



4 In Thynnus vulgaris and Thynnus brachypterus the veins from the stomach, the 

 intestinal canal and the spleen, before entering the liver, form very large retia mira- 

 bilia of pencil-shaped branches, from which subsequently larger veins arise as portal 

 branches ; the artery that goes to the abdominal viscera \arteria codiaca) distributes 

 its blood by such nets alone to the stomach and the intestinal canal, but to the liver by 

 an hepatic artery not deviating from the ordinary form. D. F. ESCHRICHT u. J. 

 MUELLER Ueber die arteriosen und venosen Wundcrnetze an der Leber des Thunfisches 

 (Abh. der ATcad. der Wissensch. zu Berlin, 1835). In Lamna cornubica such arterial 

 and venous networks are met with, here formed by the hepatic veins ; in Carcharias 

 vulpes there are venous and arterial retia m. of a pennate form on the stomach and that 

 part of the intestinal canal where the spiral valve is situated : see A. EARTH Diss. inaug. 

 de Eetibus mirabilibus, Berolini, 1837, 4 to > af ter observations of J. MUELLER. 



