A DEGENERATE GLAND. — PRINCE. 217 



non-essential character. Its form and relations may become 

 most surprising in character. Its minute structure may show 

 the most perplexing- variation. The delicate membrane which 

 forms the silvery tubular sac in the herring {Clujyea) Plate 20, 

 iig. 3, sb., or the capacious ovate organ attached to the inner 

 surface of the dorso-lateral body-wall in Perca, Plate 20, tig. 

 3, sb., is in great contrast to the thick massive-walled swim- 

 bladder of the sturgeon (Sturio) (Plate 23, figs. 1, and 2), or 

 the spong}^, complexly meshed and vascular organ seen in the 

 Bowfin (Amia calva). Special muscles occur in the walls. 

 They are usually striped i. e. voluntary, and under the control 

 of the fish ; but the duct possesses unstriped muscle, excepting 

 in the rare cases where a sphincter muscle is present for closing 

 and opening the entrance to the duct. Humboldt thought that 

 the muscles of the swim-bladder efiected its compression, and 

 aided in the descent or ascent of fishes in the water, an idea 

 which M. Delaroche (No. 6) admitted, might be possible 

 though his own researches rather pointed, not to effecting by 

 the swim-bladder such a change in the specific gravity of the 

 fish as compared with the surrounding water, but rather to 

 keeping it at the same specific gravity as the watery environ- 

 ment and presenting the fish rising or sinking. M. Morcau, 

 in his later researches (No. 16) confirms Delaroche's view and 

 adds that the equilibration is due to the external pressure of 

 the water upon the body walls, and he emphatically contradicts 

 Humboldt's idea and asserts that the muscles on the walls of 

 the swim-bladder are not used to regulate or alter the volume 

 of that organ. 



In general the walls of the swim-bladder exhibit four 

 layers in the adult fish viz. : (1) a "esselated lining 

 epithelium ; (2) an inner fibrous layer, often silvery in 

 appearance, due to wavy fibres or to bright crystalline rods ; 

 (3) an outer thick fibrous layer, yielding isinglass, and especially 

 dense in Sturio ; (4) a very delicate muscular stratum, which 

 may be so arranged as, it is claimed, to compress and drive the 



