ON THE AIR-BLADDERS OF FISH. 101 
which is usually in its longitudinal axis. In some forms, as in 
Holocentrum and Sargus, ccecal processes pass inwards to attach 
themselves to a membrane closing in a portion of the internal 
ear. The varieties of air-bladders in the Physoclisti are 
exceedingly numerous, but it is nof my purpose to refer to such 
in this place. 
Among the Physostomi, or those families of fish in which a 
pneumatic tube continues pervious throughout life, we find the 
majority are fresh-water forms, placed intermediate between the 
Physoclisti and the Ganoids. This pneumatic tube possesses the 
same coats as the air-bladder, an external, fibrous and tough, and 
an internal, which is mucous and vascular: its length is various, 
while it is said to be occasionally tortuous. It opens, as a rule, 
on the dorsal wall of the alimentary canal, or, as in the Herring 
and its allies, at the terminal extremity of the stomach. But one 
form among the Characinide, the Erythrinus, is a most 
interesting link between Physostomus ‘leleosteans and _ air- 
breathing Ganoids: in this genus, although the air-bladder is 
above the alimentary canal, the pneumatic tube passing down- 
wards pierces the left side of the throat. The air-bladder is 
likewise divided by fibrous partitions, but whether such are 
exceedingly vascular or not, whether this fish uses this organ for 
respiration or simply for flotation, there does not appear to be any 
information on which to decide. 
It seems to be the rule, so far as I have yet been able to 
ascertain, that the fresh-water Physostomi have a connection by 
means of a chain of ossicles between the air-bladder on one side 
and a process of the internal ear on the other. Instead of a tube 
filled with gas passing inwards from the air-bladder, as in some 
Physoclisti, to the base of the skull, the connection is entirely of 
an osseous character. In the Cyprinidé it is more or less as 
follows :—Three ossicles on either side connect the outer surface 
of the air-bladder with the atria of the labyrinth, consequently 
any dilatation or compression of the contained gas must have 
some effect upon the movement of this chain of bones. ‘These 
auditory ossicles were first pointed out by Weber, and since his 
time it has been shown that they pertain, like the capsules of the 
special organs of sense, to the splanchno-skeleton. 
In the same family of Carps we find, in the East Indies, a 
curiously modified form of Loach, Botia. In its habits it can 
