AIR-BLADDER OF FISHES. 491 



vein of the spleen always contributes to form the ' vena porta? ; ' 

 but it is important to note that it is not essential to the formation 

 of that vessel. The absence of the spleen in fishes is concomitant 

 with the absence of the pancreas ; and the increased size and 

 complexity of the spleen is associated in some fishes with a cor- 

 responding developement of the pancreas. Thus there is an 

 accessory spleen in the Sturgeon ; and the spleen is divided into 

 numerous distinct lobules in Lamna, Selache, fig. 278, d, and 

 some other highly organised Plagiostomes. In most Osseous 

 Fishes the spleen is appended by its vessels, and a meso-splenic 

 fold of peritoneum to the hinder end or bend of the stomach, or 

 to the beginning of the intestine : it is of variable but commonly 

 triangular shape ; of a deep red or brown-red colour, and soft 

 and spongy : the venous cells of which it is chiefly composed are 

 filled with granular corpuscles. 



§ 86. Air-bladder of Fishes. — The organ so denominated is found, 

 in most Osseous Fishes, in the form of an elongated bladder, tensely 

 filled by air, extending along the back of the abdomen, between the 

 kidneys and the chylopoietic viscera, fig. 281, k, and sometimes 

 (Gymuotus, fig. 233, d, Ophiocephalus, Coius) beneath the caudal 

 vertebras to near the end of the tail. It is sometimes bifurcate (as 

 we see it in most Scomberoids and Carangoids, 1 in some species of 

 Diodon, Tetrodon, of Dactylopterus, Pimelodus, Prionotus) ; seldom 

 divided lengthwise into two bladders (Arius, Gagora, Polypterus, 

 Lepidosiren, fig. 324, p, p) : more often divided crosswise into 

 two compartments, which intercommunicate by a contracted 

 orifice (Cyprinidce, fig. 229, p q, Characinidce), or are quite 

 separate (Bagrus filamentosus , Gymnotus equilabiatus). In the 

 Siluroid genus Pangasius the air-bladder is divided into four 

 longitudinally succeeding portions. In the Trigla hirundo the 

 swim-bladder is notched anteriorly by one indent, and posteriorly 

 by two indents, from which notches septa project inwards : some- 

 times the air-bladder is divided partially, both lengthwise and 

 crosswise (Cobitis fossilis, Auckenipterus furcatus, some species of 

 Pimelodus). Sometimes the bladder sends forward two blind 

 processes from its forepart (Sphyrcena barracuda, Trigla cuculus, 

 Conodon antillanus, some species of Micropogon and Otolithus) ; 

 sometimes from its hind part {Cantharis vulgaris, Lethrinus 

 atlanticus, Heliases insolatus, some species of Sillago, Mcena, and 

 Smarts) ; sometimes from both ends (Dules maculatus, Pimelipterus 



1 Dr. Giinther tells me that all the species of these families with a short and 

 elevated body, with a short abdominal cavity, and with strong first haemal and inter- 

 haemal spines, have the air-bladder bifurcate behind, extending backward between the 

 muscles of the tail, to or beyond the middle of its length. 



