506 PEOr. T. W. BEIDGE ON THE 



The anterior or pre-coelomic section of the air-bladder consists 

 posteriorly of a somewhat subsplierical sac (sb.s.), separated 

 externally from the abdominal portion of the org.in by a shallow- 

 transverse or obliquely disposed groove. In this groove the 

 subvertebral portion of the mesonephros curves downwards on 

 each side to fuse with the large anterior and unpaired portion 

 of that organ, or " head-kidney," which is situated dorsad to the 

 heart and gills, but ventrad to the subspherical sac. Anteriorly, 

 the sac abruptly contracts to form a somewhat tubular or slightly 

 inflated, median prolongation {t.p.), which extends forwards 

 immediately above the dorsal extremities of the hinder branchial 

 arches, in contact with the ventral surfaces of the centrum of 

 the first vertebra and the ba?ioccipital, and by each of its lateral 

 surfaces is in relation with the branchial branches of the 

 corresponding pneumogastric nerve. A constriction separates 

 the subspherical sac from its tubular prolongation, and at the 

 same time transmits on the left side the coeliac branch of the 

 dorsal aorta. Eventually the tubular prolongation divides into 

 two somewhat narrower, csecal, auditory cornua, each of which 

 (I.a.c.) diverges somewhat from its fellow and passes upw^ards and 

 outwards, as well as forwards, in order to reach the outer surface 

 of the auditory capsule of its side, where, as Cuvier and Valen- 

 ciennes (op. cif. p. 140) have described in the case of Notopterus 

 Pallasii, it becomes intimately associated with the enclosed mem- 

 branous labyrinth of the organ of hearing. Between the origins 

 of the two auditory caeca, and on the dorsal side, the dorsal aorta 

 may be seen passing backwards to reach the bcmy aortic groove, 

 to which further reference will subsequently be made. 



The whole of the anterior section of the air-bladder is situated 

 in front of the abdominal cavity, and consequently none of its 

 various divisions or chambers are invested externally by the 

 peritoneum. 



It is perhaps worthy of note that anteriorly and dorsally the 

 branchial cavity is prolonged forwards on each side of the skull 

 for some distance, parallel to the lateral surface of the auditory 

 capsule, and directly external to the corresponding auditoiy 

 cornua of the air-bladder. This singular extension of the 

 branchial cavity practically takes the form of a csecal diverticulum 

 (PI. 36. fig. 1, a.b.c), ending blindly in front by reason of the 

 gradual contraction of its walls, but communicating behind with 

 the general branchial cavity. Into each csecal di\erticulum the 



