272 FISHES CHAP. 



largest, the liver arises as a caecal outgrowth from the embryonic 

 mesenteron, and in this primitive stage recapitulates a condition 

 which is retained throughout life in Amphioxus. By the sub- 

 sequent division and branching of this outgrowth the massive 

 compound tubular gland of the adult Fish is eventually formed. 

 The liver of Fishes (Figs. 153, 154) is very variable in size, 

 shape, colour, and degree of lobulation. Anteriorly, it is usually 

 moulded to the posterior face of the transverse septum between 

 the pericardial and abdominal portions of the coelom, and from 

 thence extends backwards in the abdominal cavity to a varying 

 distance, in some Sharks as far as the cloaca. Externally, the 

 gland is invested by the peritoneum, which extends on to it from 

 the pericardial septum and forms a suspensory fold, and also from 

 the oesophagus and stomach. The shape of the liver usually 

 bears some relation to that of the body, being, for example, 

 longest in the Eels and broadest in the Eays. In the great 

 majority of Fishes the liver is bilobed, consisting of two sub- 

 equal lateral lobes, disposed longitudinally and confluent anteriorly 

 for a portion of their extent. From this normal type there are 

 a few minor variations.^ In Petromyzon, Lepidosteus (Fig. 155, 

 B), and a few Teleosts (e.g. the Gymnodontes, Lophobranchii, and 

 some Salmon] dae) the liver is unilobed. In the Myxinoids and 

 in the Dipnoi (e.g. Protopterus), the organ is bilobed, but the 

 small anterior lobe lies immediately in front of the much larger 

 posterior lobe, with the gall-bladder between the two (Fig. 155, 

 A). In some Teleosts (e.g. Scomber), the liver is trilobed. A 

 gall-bladder is invariably present in either the larval or adult 

 Cyclostomata, in the Chrondrostei, Holostei, Grossopterygii and 

 Dipnoi, and generally also in Elasmobranchs and Teleosts. In 

 the Elasmobranchs it is rarely entirely wanting, as in Sphyrna 

 and Pristis, and in the Teleosts in some of the Gurnards (Trigla). 

 The gall-bladder and bile-duct of Petromyzon fiuviatilis atrophy 

 after the metamorphosis which follows the larval Ammocoetes stage, 

 but in Petromyzon marinus the duet, although usually absent, is 

 sometimes retained. In the Ammocoetes the epithelium lining 

 the gall-bladder is ciliated. In some Fishes, as, for example, in 

 many Elasmobranchs, the gall-bladder is more or less completely 

 embedded in the substance of the liver; in others, as in most 

 Teleostomi, the organ is quite distinct from the gland (Fig. 154). 

 ' Stannius, HaivXhlc. d. Zool., Berlin, 1854, ii. p. 201 ; Owen, op, cit, p. 425. 



