_Comparative Anatomy._--In the Acrania (Amphioxus) the liver is probably represented by a single ventral diverticulum from the anterior end of the intestine, which has a hepatic portal circulation and secretes digestive fluid. In all the Craniata a solid liver is developed. In the adult lamprey among the Cyclostomata the liver undergoes retrogression, and the bile ducts and gall bladder disappear, though they are present in the larval form (Ammocoetes). In fishes and amphibians the organ consists of right and left lobes, and a gall-bladder is present. The same description applies to the reptiles, but a curious network of cystic ducts is found in snakes and to a less extent in crocodiles. In the Varanidae (Monitors) the hepatic duct is also retiform (see F. E. Beddard, _Proc. Zool. Soc_., 1888, p. 105). In birds two lobes are also present, but in some of them, _e.g_. the pigeon, there is no gall-bladder. [Illustration: FIG. 4.--Diagrammatic Plan of the Inferior Surface of a Multi-lobed Liver of a Mammal. The posterior or attached border is uppermost. u, Umbilical vein of the foetus, represented by the round ligament in the adult, lying in the umbilical fissure. dv, The ductus venosus. vc, The inferior vena cava. p, The vena portae entering the transverse fissure. llf, The left lateral fissure. rlf, The right lateral fissure. cf, The cystic fissure. ll, The left lateral lobe. lc, The left central lobe. rc, The right central lobe. rl, The right lateral lobe. s, The Spigelian lobe. c, The caudate lobe. g, The gall bladder.] [Illustration: FIG. 5.--Human Liver showing a reversion to the generalised mammalian type.]