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 mirahile, which, connected with a 

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

 the choroid gland lately spoken of, and which usually occurs simul- 

 taneously with the false gilP. 



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

 deserve to be regarded as serving for respiration'^. 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 epitlielimn 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 fissm-e, as a 

 species of glottis, which leads immediately from the cesophag-us 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 Akad. der WissenscJi. zu Berlin, aus d. Jahre 

 1839, s. 213 — 240, 247 — 261. 



^ There are very numerous publications on this organ, of which we are ciontent 

 to notice the following : Ct. Fischer Versuch iiher die Schwimmblase der Fiscke, 

 Leipzig, 1 795, 8vo ; De La Roche Observations sur la vessie aerienne des Poissons, 

 Ann. du Mus. xiv. 1809, pp. 184 — 217, pp. 245 — 289 ; H. Rathke Bemerhungen iiber 

 die ScMoimmblase einiger Fische in his Beitrdge zur Gesch. der Thierwelt, 4te Abth. 

 1827, s. 102 — 120, and his later investigations in Mueller's Archiv, 1838, s. 413 

 —445, Taf. 12; K. E. Von Baer Untersuchungen iiber die Entwickelungsgesch. der 

 Fische, nebst einem Anhange iiber die Schtvimmblase, Leipzig, 1835, 4to; H. S. R. 

 Jacobi De vesica ae'rea Piscium, Diss, inaug. Berolini, 1840, 4to, under the auspices 

 of J. Mueller. 



