AIR-BLADDER 307 



in the Physoclisti. In the Herring (Clupea liourengus}} in 

 addition to the proper ductus, which is connected with the 

 distal end of the caecal stomach, a tubular canal leaves the 

 hinder extremity of the bladder and opens externally on the left 

 side of the genital aperture ; consequently, in this Fish the air- 

 bladder has a secondary and direct connexion with the exterior 

 in addition to the primary and indirect comnmnication by means 

 of its proper duct. The Horse-Mackerel (Caranx trachurus) 2 is 

 even more peculiar. This Teleost has no true pneumatic duct, 

 but instead a special duct which passes from the bladder to open 

 into the right branchial cavity by a very minute aperture. In 

 neither case is anything known of the mode of origin or morpho- 

 logical nature of the secondarily acquired duct. 



(/) The air-bladder differs greatly in its degree of vascularity 

 in various Teleosts, as well as in the extent to which its capil- 

 lary blood-vessels accumulate at special points on the inner 

 surface to form the so-called " red bodies " or " red glands." 

 In some Teleosts the distribution of capillaries is uniform or 

 nearly so ; in others, as in the Carp (Gyprinus carpio] the vessels 

 are arranged in fan-like, radiating tufts over almost the whole 

 extent of the inner surface ; in others again, as in the Pike 

 (Esox Indus') the tufts are larger and more definitely localised. 

 A more extreme modification occurs in some of the Physostomi, 

 in which a remarkable concentration of capillaries takes place 

 at one or more points on the inner surface of the bladder, which 

 project into the cavity of the organ in the form of variously 

 shaped blood -red masses. These "red bodies" are essentially 

 retia mirabilia, consisting of masses of interlacing, tightly- 

 packed capillaries with their afferent arteries and efferent veins. 

 The flattened lining epithelium of the bladder is continued over 

 these bodies without undergoing any special modification. In the 

 common Eel (Anguilla vulgaris) there are several of these bodies, 

 of which the largest are near the entrance of the pneumatic duct. 



In the Physoclisti the " red bodies " seem to be replaced by 

 true glands, 3 which nevertheless in appearance closely resemble 

 the former. Some of the Gadidae, such as the Cod (Gadus 



1 Weber, De aure et auditu Hominis et Animalium, Leipzig, 1820, p. 73. 



2 Moreau, Compt. Reiid. Ixxx. 1875, p. 1247. 



3 Coggi, Mitth. Zool. Stat. Neapel, vii. 1887, p. 381 ; Swale Vincent and Stanley 

 Barnes, Journ. Anat. and Phys. xxx. 1896, p. 545. 



