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 lfe 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 Rays. 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, Lophobranchu, and 
some Salmonidae) 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, Crossopterygii 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 (7vrigla). 
The gall-bladder and bile-duct of Petromyzon fluviatilis 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 lning 
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). 
1 Stannius, Handbk. d. Zool., Berlin, 1854, ii. p. 201; Owen, op. cit. p. 425. 
