218 THE SWIM-BLADDER OF FISHES 



contents of the posterior chamber into the anterior 

 chamber of the swim-bladder. The internal vascular structures 

 are very simple and confined to one portion of the swim -bladder 

 as in the conger, (Plate 22, fig;. 1), which exhibits two red 

 rounded vascular bodies (a.a.) situated near to the entrance of 

 the pneumatic duct, or the blood-vascular network is spread 

 over a large portion of the internal surface forming, as in the 

 haddock, a dense reticulum of blood-vessels, or again these retia 

 tnirabilia may be wholly absent. In size the swim-bladder 

 vanes exceedingly, it may be small as in many of the herring 

 family but simple in form, (Plate 20, fig. 3, sb.) It may as in 

 the common perch {Perca flavescens) be still simple but of 

 larger capacity as compared with the size of the fish, (Plate 20, 

 fig. 2, sh.) Thus in a 10 inch perch the swim-bladder is no less 

 than 4 inches long and nearly an inch in diameter, while in a 

 perch 7^ inches long the organ is 3 inches in length. Its volume 

 is not more variable than its shape and relations to other organs 

 in the fish possessing it. In very young post-larval fishes it 

 forms a prominent feature, glistening like a silvery gaseous 

 bubble of elongated shape through the translucent body (see 

 figure of Gunner \ inch long, Plate 21, fig. 13, and Plate 23, 

 figs. 3. 4, 5, 6.) Instead of the simple ovate sac of the Perch 

 and Gunner, it may become extremely elongated as in the 

 Herring sending forward a pair of delicate ducts, which end in 

 two swollen sacs in the skull, close to the ear, one on each side, 

 or as in the Haddock or God ceasing all connection with the 

 alimentary canal, the capacious sac may continue forwards as 

 two solid vermiform cornua terminating near the ears, (Plate 20, 

 fig. 1, corn.) In the Carp the pneumatic duct not only persists 

 but the anterior sac of the two-chambered swim-bladder may 

 directly connect with the vestibular sac of the ear by ligaments 

 and a chain of three bones, (Plate 20, fig. 4, i. k. I.), really costal 

 modifications, the last and longest being attached to the swim- 

 bladder, (sb.). In the Siluroids the relations of the two organs 

 are far more complex, for the anterior vertebrae may coalesce 



