ATR-BLADDER ]N KOTOPTEEUS BOEXEEKSIS. 507 



dorsal portions of the first and second branchial arches (hr}, br."}, 

 with their branchial lamellae, extend for some distance ; the 

 remaining arches, however, lying posterior to the opening of the 

 diverticulum into the dorsal portion of the branchial cavity. 

 The relatively thin inner wall of the branchial diverticulum is 

 coextensive with, aod directly and closely iuvests the outer wall 

 of, the corresponding auditory cseeum (l.a.c), and hence in this 

 region the air-bladder and its gaseous contents are more intimately 

 related to the external medium in which the Fish lives than 

 is the case at any other point. 



Por a Teleost the ductus pneumaticus (text-fig., and PI. 36. 

 fig. 3, d.p.) is remarkably short and unusually wide, its length 

 not exceeding 3 mm., while its lumen, even in a spirit-preserved 

 specimen, is approximately the same in diameter. The oesophageal 

 opening of the ductus is in the mid-dorsal line, and about 8 mm. 

 behind the last branchial cleft. The a[;erture by which the 

 ductus communicates with the air-bladder is situated a little to 

 the left of the median ventral line, and directly posterior to the 

 oblique groove (o.y.) separating the subspherical sac from the 

 abdominal portion of the organ. The ductus is surrounded at its 

 oesophageal extremity by a strong sphincter muscle, but no valvular 

 mechanism in connection with either of its apertures could be 

 detected. 



No gas-secreting or gas-absorbing "red-glands" or "red- 

 bodies " were to be found in the air-bladder. 



III. The Ifteexal Structcee, and the Relations and 

 Attachments of the Aie-bladdee to the Skeletoit. 



In the condition of their inner or mesial walls, and in the 

 relation of these structures to the skeletal elements of the tail, 

 the caudal prolongations of the air-bladder present several 

 interesting features. 



With regard to the character and disposition of the caudal 

 skeletal elements, it may be mentioned that the haemal spines of 

 the suprajacent vertebrae are relatively short (PI. 38. fi:g. 2, h.s.). 

 The radial elements of the anal fin (r.e.), on the contrary, are of 

 considerable length in the anterior part of the tail, although 

 they gradually become much shorter as they are traced backwards 

 towards the caudal fin ; and as two of them, forming a pair, are 

 associated dorsally with each haemal spine, it follows that the 

 radial elements are twice as numerous as the caudal vertebrae, at 



