AIR-BLADDER ]]S NOTOPTERUS BORXEE>'SIS. 505 



occupies the usual position beneath the vertebral column, being 

 separated from the latter by the kidney. Tiie ventral surface 

 only is invested by the peritoneum, the line of reflection of this 

 membrane on to the inner surface of the abdominal wall being 

 coincident with the junction of the lateral with the ventral wall 

 of the organ. Posteriorly, the peritoneum is reflected down- 

 wards on to the haemal arch and spine of the first caudal vertebra 

 and the first radial element (" interspinous bone " or " pterygio- 

 phore") of the aual fin, and therefore does not accompany the 

 caudal prolongations of the air-bladder. 



At its binder extremity, immediately anterioi" to the first 

 radial element of the anal fin, the abdominal portion of the 

 bladder divides into two caudal caeca {l.c.c), each of which at its 

 commencement, if not so wide, is nearly twice the vertical dimen- 

 sion of the former. The two caeca extend backwards on opposite 

 sides of the tail, and, gradually contracting, finally terminate in 

 pointed and almost filiform extremities about 13 cm. behind the 

 anus, or, approximately, about the middle of the length of the 

 tail. In its course along the tail each caecum is situated wholly 

 internal to the caudal musculature of its side, and m immediate 

 contact with the haemal spines of the caudal vertebrae and the 

 supporting radial elements of the extensive anal fin. The dorsal 

 and external walls of each caecum are invested by a strong 

 aponeurotic membrane, the outer surface of which receiver the 

 insertions of the inner margins of the fibrous septa separating 

 the caudal myotomes. 



Along nearly the whole length of each of the caudal prolonga- 

 tions of the air-bladder, the ventral margin gives ofl" a fringe of 

 numerous short but relatively wide diverticula (v.d.), which form 

 pairs with their fellows of the opposite side of the tail. With 

 the exception of the first two of the series, each diverticulum 

 eventually terminates by subdividing into an anterior and a 

 posterior slender, filiform caecum {f.c), which extend ventrally 

 towards the base of the anal fin, and are intercalated between, 

 and partially covered by, the superficial flexor muscles of the 

 contiguous dermal fin-rays, but are external to the corresponding 

 deep flexors. The second diverticulum has three filiform caeca, 

 while the first remains simple and undivided. The series of 

 Ventral diverticula and their filiform prolongations gradually 

 diminish in size from before backwards, and eventually cease at 

 some little distance anterior to the tapering terminal pjrtions of 

 the caudal extensions of the bladder. 



