350 On a remarkable hind of Air-bladder. 



the complicated air-bladder of ColUchthys lucida (see Catal. 

 Fish. ii. p. 313) ; but its ramifications are shorter, and do not 

 envelope the abdominal viscera. 



The organ as a whole is lanceolate, leaf-shaped, thickest 

 along the middle, and gradually becoming very thin towards 

 the margins. Its greatest length is 5| inches, and its greatest 

 width 2 inches. In the collapsed state its thickness in the 

 middle is about 3 lines, and may have been about 5 lines 

 when expanded by air. On its visceral surface we distinguish 

 the body of the organ, with a smooth, polished, pearl-white 

 surface ; it is elongate, only 7 lines wide in the middle, with 

 its anterior extremity rounded, and with the posterior tapering 

 into a very fine tube. The membrane is thick and stiff, and 

 can be readily divided transversely into strips, each of which 

 corresponds to one of the lateral branches. The body emits 

 on each side fifty-two branches or tubes, communicating by a 

 small opening with the cavity of the body of the air-bladder, 

 and split into secondary and tertiary smaller branches running 

 towards the margins of the organ. All these branches, as 

 seen on the visceral surface, are connected by a cellular tissue, 

 which can be easily severed with the point of a needle. On 

 the dorsal surface nothing is visible of the main branches, 

 but only the fine terminations of the secondary and tertiary 

 branches appear, the whole resembling a thick network of 

 fine fibres, of which the central ones are short, running in a 

 backward direction, whilst the lateral are longer and diverge 

 towards the margins. 



Such is the general appearance of the air-bladder on its 

 visceral and dorsal surfaces. However, to understand the 

 ramification of the branches, it is necessary to isolate one or 

 several by dissection. It is then seen that each branch forms 

 a kind of lamina, its ramifications lying in the same plane, 

 and being connected with one another in the same manner as 

 the branches themselves. Each branch bifurcates immediately 

 after its egress from the body into a visceral and dorsal stem. 

 The visceral stem bifurcates twice or thrice again ; and its 

 terminal tubes are the longest, reaching the margin of the 

 organ. The dorsal stem is bent over towards the median 

 line of the dorsal surface of the organ, and emits a number 

 of bifurcating branchlets, which are the shorter and thinner 

 the nearer they are to the median line; and their terminations 

 are seep in the middle part of the dorsal surface, as described 

 above. 



EXPLANATION OF PLATE XVIII. 



Fiff A Visceral surface. 



Fiij. H Dorsal surface. 



Fig Vn isolated branch : a, dorsal stem ; b, visceral stem. 



