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scattered portions of the gland are very visible through the transpa- 

 rent parietes of the bladder. Each branch of any considerable size 

 which is given off from the main artery supplying the bladder, ends 

 in three or four of the brush-shaped appendages, as in PI. xiii. fig. 4, 

 and even in the uninjured state they appear to the unassisted eye as 

 so many clusters of Vorticella, or bell polyps. The arrangement in 

 the other part of the interior of the bladder is of the parallel kind be- 

 fore described as occurring in other fishes, but the muscular structure 

 of the bladder appears but little developed, as its parietes are very 

 transparent. 



In the eel, the air-bladder is very remarkable ; and this is the only 

 fish in which I have been able to inject satisfactorily the upper part 

 of the bladder. This part, described by most authors as communi- 

 cating with the oesophagus (PI. xiii. fig. 1 a), is exceedingly thin, and 

 the vessels which ramify in its interior, are very visible through its 

 parietes. It joins the second portion of the bladder at a very acute 

 angle about its middle (PI. xiii. fig. lb). This last portion is spindle- 

 shaped, and much thicker than the first, and is invested with several 

 layers of a silvery membrane, which renders it opaque. At the point 

 of junction of these two compartments we have two glandular bodies, 

 placed one on each side of the duct of communication between the 

 compartments, (Plate xiii. fig. 1 d) ; these are compact and hard, and, 

 like the same parts in the cod fish, are composed entirely of parallel 

 vessels, and with them are connected all the trunks which supply the 

 posterior compartment of the bladder. The blood is received from 

 the aorta very high up, and is first distributed to the upper compart- 

 ment, then it goes to the two glandular bodies before spoken of, and 

 from them is distributed to the whole interior of the posterior portion 

 of the bladder, and returned again by a large vein, which may be very 

 well seen in the thin membrane, running in a direction nearly paral- 

 lel to the artery. The arrangement of vessels in the upper compart- 

 ment is very peculiar, and approaches nearer to that of the cellular 

 lungs of the Reptilia than any other system, in which animals we have 

 a single twig, dividing into net-work of nearly equal-sized capillaries, 

 the interspaces or meshes being also remarkable for the equality of 

 their size. In the posterior compartment, the distribution is some- 

 what similar to that in the upper part, but the branches are far more 

 tortuous, and few if any meshes are formed, (PI. xii. fig. 4); and both 

 arteries and veins have the same arrangement, and run parallel to 

 each other, and when one system of vessels is successfully injected, 



