XI AIR-BLADDER 307 
in the Physoclistii In the Herring (Clupea harengus)) 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 communication by means 
of its proper duct. The Horse-Mackerel (Caranx trachurus)” 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. 
(7) The air-bladder differs greatly in its degree of vascularity 
in various Teleosts, as well as in the extent to which its capuil- 
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 (Cyprinus carpio) the vessels 
are arranged in fan-lke, radiating tufts over almost the whole 
extent of the inner surface; in others again, as in the Pike 
(Esox lucius) 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,® which nevertheless in appearance closely resemble 
the former. Some of the Gadidae, such as the Cod (Gadus 
Weber, De aure et auditu Hominis et Animalium, Leipzig, 1820, p. 73. 
Moreau, Compt. Rend. 1xxx. 1875, p. 1247. 
Coggi, Mitth. Zool. Stat. Neapel, vii. 1887, p. 381; Swale Vineent and Stanley 
Barnes, Journ. Anat. and Phys. xxx. 1896, p. 545. 
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