34 



CLASS XIV. 



must not be confounded which in many fishes are found on the 

 palate, above and outwards from the gills, sometimes presenting a 

 more glandular lobulated structure, but mostly a pennate or pecti- 

 nate form with a single row of leaflets. Since these organs receive 

 arterial blood, they cannot be for respiration. The blood that 

 returns from them unites with that of an arterial branch (arteria 

 ophthalmica magna) which supplies the choroid coat of the eye-ball, 

 where usually it forms a rete mirabile, which, connected with a 

 similar venous net for the blood returning from the eye-ball, forms 

 the choro'id gland lately spoken of, and which usually occurs simul- 

 taneously with the false gill 1 . 



As little as this organ does the swimming-bladder of fishes 

 deserve to be regarded as serving for respiration 2 . This bladder 

 occurs in the sturgeons and many osseous fishes, and is situated 

 above the intestinal canal towards the spine, but under the kidneys, 

 almost always as an unpaired symmetrical organ. Its walls are 

 formed by two membranes ; an external tendinous membrane and 

 an internal thin mucous membrane, richly supplied with vessels 

 and covered with flat epithelium on the inner surface. In addition, 

 this bladder is invested on the ventral surface with a production 

 from the peritoneum. In some fishes a duct proceeds from it to the 

 oesophagus or to the stomach ; in some there is a fissure, as a 

 species of glottis, which leads immediately from the oesophagus to the 

 swimming-bladder. Since the swimming-bladder comes into being 

 as an eversion of the intestinal canal, it is probable that even in cases 

 where it is quite closed a canal existed at an earlier period, which 

 has been condensed to form a ligament, or has been entirely absorb- 

 ed. In many fishes, in most sea-fishes especially, the swimming- 



1 Compare J. MUELLER Abhandl. der Alcad. der Wissensch. zu Berlin, aus d. Jahre 

 1839, s - 213240, 247261. 



2 There are very numerous publications on this organ, of which we are content 

 to notice the following : G. FISCHER Versuch uber die Schwimmblase der Fische, 

 Leipzig, 1795, 8vo ; DE LA EOCHE Observations sur la vessie aerienne des Poissons, 

 Ann. du Mus. xiv. 1809, pp. 184 217, pp. 245 289 ; H. RATHKE Bemerlcungen uber 

 die Schidmmblase einiger Fische in his Beitrage zur Gesch. der Thierwelt, 4te Abtl 

 1827, s. 102 1 20, and his later investigations in MUELLER'S Archiv, 1838, s. 41; 

 445, Taf. 12; K. E. VON BAER Untersuchungen uber die Entwiclcelungsgesch. 

 Fische, nebst einem, Anhange uber die Schwimmblase, Leipzig, 1835, 4to; H. S. R. 

 JACOBI De vesica aerea Piscium, Diss. inaug. Berolini, 1840, 4 to, under the auspic 

 of J. MUELLER. 



