A DEGENERATE GLAND. — PRINCE. 211 



the most part). It is difficult to see why trout, carps, pike, 

 sticklebacks, and sturgeon, frequenting shallows, where a 

 hydrostatic organ is comparatively useless and unnecessary, are 

 provided with it, while the Sand-launce (Ammodytes), the 

 lump-lish {Cyclopterus), or the mackerel, should be without it. 

 Its erratic occurrence shows how unessential it is to fishes now, 

 however important it may primitively have been. 



It is in the early condition, as seen in the embryonic and 

 larval stages of fishes possessing this organ, that the swim- 

 bladder is most interesting and suggestive. It has all the 

 features characteristic of a large but simple gland. Krause 

 distinguishes (No. 12, p. 206) four parts in a simple gland (1) 

 the mouth, (2) the neck, (3) the body, (4) the ccecal extremity, 

 and these may all be noted in the early swim-bladder, which is 

 really a blind sac provided with a duct, and usuallj^ having an 

 abundant blood-supply. Glands are always w^ell supplied with 

 blood, as their function is to secrete some characteristic substance 

 from the blood passing through their vascular net-work. The 

 swim-bladder arises, in all cases, as a dorsal bud or po ket of 

 cells on the upper wall of the hind portion of the oesophagus. 

 (Plate 1, figs. 21 and 2). At the earlier stages the gullet cannot 

 be clearly marked off from the mesenteron or stomach. Thus, 

 in a haddock, a few days before hatching, an evagination from 

 the dorsal "J^ide of the alimentary canal is observed projecting 

 perpendicularly from the centre of the gullet. (Plate 21, fig. 4 

 sb). In the same microscopic section shown in the plate, a 

 ventral diverticulum also appears, viz., the rudiment of the liver 

 (I). The section was in a slightly oblique transverse vertical 

 plane, and therefore includes both organs. The lengthening of 

 the alimentary canal, owing to the rapid growth of the young 

 fish, soon more widely separates the swim- bladder and the liver, 

 and, it may be pointed out, causes the straight canal to twist or 

 curve to one (the left) side, and the balloon-shaped bladder 

 curves over to the right, (Plate 21, fig. 5, sh). For some days 



Proc. & Trans. N. S. Inst. Sci , Vol. XI. Trans.— O. 



