310 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
is this : that the curious and, in many respects, unique persistence of a useless air- 
bladder is due to its highly specialized connection with the Weberian mechanism. 
As is well known, comparative anatomy affords numerous examples of the persistence 
of functionless but harmless skeletal structures, which occasionally may even retain 
traces of their softer connections, and, in our opinion, it is the retention of the 
Weberian ossicles that has been the principal cause of the persistence of a useless 
air-bladder, in much the same way that the useless and rudimentary bones of the hind 
limbs of certain aquatic Mammalia sometimes retain remnants of their equally useless 
muscles and ligaments. 
The second problem suggested, viz., the frequent encapsulation of the rudimentary 
air-bladder by bone in abnormal Siluridm, is a veiy difficult one, and one which 
we must confess our inability to explain satisfactorily, or to do more than suggest 
possible solutions. Three points may be kept in view in discussing this question - 
(l) That the degree of encapsulation generally bears some relation to the extent 
to which the air-bladder retains the structural integrity of its walls and its apparently 
normal connection with the tripodes (2) that however defective in some respects 
the individual Weberian ossicles may be, the continuity of those that persist is 
complete in all Siluridae abnormales ; and (3) that encapsulation is strictly confined to 
the rudimentary air-bladders of the Ostariophyseae, and occurs in no other living 
Teleostei.f 
Partial encapsulation, to the extent that the reduced air-bladder occupies trans- 
versely disposed grooves or hollows on the ventral surfaces of the expanded and 
conjoined transverse processes of the fourth and fifth vertebrae, is, perhaps, not 
difficult to explain, for reduction in the size of the anterior cliamber has sinqjly been 
acconq:>anied by a corresponding contraction and curvature of the modified transverse 
processes which normally closely invest and are adapted to the convexity of its dorsal 
and anterior surfaces. The difficulty is felt to be greater in the case of such Siluroids 
as Cetopsis, Clarias, Calloniysta.r, Aca'/ithicus, and the various Loricaroid genera, in 
which the diminutive air bladder is almost completely encapsuled by bone, and while 
obviously iiseless for all hydrostatic purposes, r-etains much of its structural integrity 
as well as its normal relations to the Weberian ossicles. In several of these genera, 
in additioii to the share taken by the transverse processes, which of itself might be 
readily accounted for in the way suggested above, certain additional means are 
employed to ensure the more complete enclosure of the air-bladder within osseous 
capsules, and this at least suggests the possibility that some specific and useful object 
is to be gained thereby. Of these additional methods we may again refer to the 
])artial ossification of the walls of the air-bladder itself {c.g., Acmithicufi) , and to the 
* Sc(‘ i\lor])hological Smnmaiy, pp. 251, 252, ami p. 259. 
t According to Wii.i.iam!son (41) tlic aii’-bladder in certain fossil Ganoids has ossified walls, if it were 
possible it would be exti'cmcly interesting to ascei'tain if these fishes possessed any form of Weberian 
mechanism. 
