ON THE ANATOMY OF FISHES. 
293 
but has no Weberian mechanism, and in which retia mirabilia are either present or 
wanting."'' Two questions are suggested by these Fishes : (1) Are they capable, like 
the Ostariophysese, of utilizing the ductus pneumaticus as a means of pressure adjust- 
ment? (2) If so, what disadvantage is entailed upon them by the want of a 
Weberian mechanism? A negative answer to the first question renders any further 
discussion unnecessary, inasmuch as the Fishes concerned will be in the same position 
as most Physoclisti so far as pressure adjustment is concerned, but an affirmative 
response, on the contrary, will necessarily involve the consideration of the second 
question. With regard to the first point the scanty evidence available is of a con- 
tradictory character, in fact, it is not at all clear in many cases hovv far the ductus 
pneumaticus admits of free communication between the air-bladder and the exterior, in 
one species only, Murcena conger [Conger vulgaris], has it been experimentally shownt 
that gas can be eliminated through the pneumatic duct when the Fish is exposed to 
artificially diminished pressure. On the other hand, the Salmonidse are generally 
credited with the possession of an open duct, but the behaviour of one species of this 
family, the “ Kilch,” when drawn to the surface from deep water scarcely favours the 
supposition that any adjustment to reduced pressure by the mechanical liberation of gas 
can take place in this species. It is true that in this case the pressure reduction was 
considerable, although it can hardly be said to have occurred with exceptional rapidity, 
or at a greater rate than probably takes place in many Ostariophyseae during ordinary 
locomotion. Moreover, in Moreau’s experiments with such Ostariophysese as the Carp 
and Tench, the Fishes were exposed to an artificial reduction of pressure, which, if 
not so great as in the case of the “ Kilch,” was at least considerable and rapid, and yet 
throughout the experiment the gradual liberation of successive bubbles of gas enabled 
the Fish to retain perfect freedom of swimming. It is therefore difficult to avoid the 
conclusion that the “Kilch” has either no open pneumatic duct, or, if it has, that the 
duct cannot be utilized for pressure adjustment. A further item of negative evidence 
is to be found in the fact that, while retia mirabilia are invariably wanting in all 
Ostariophyseae, they are present or absent in the remaining Physostomi in much the 
same way, and to the same extent, that they are present or wanting in the Physoclisti, 
and this certainly suggests the inference that gaseous secretion and absorption are 
important factors in pressure adjustment in these Fishes. From the same point of 
view it is somewhat significant that wherever the mechanical liberation of gas has 
been proved to take place in Fishes other than the Ostariophysem it is, with one 
exception [e.g., Conger), by some other and secondarily acquired means than the 
retention of the ductus pneumaticus for the purpose. Thus Moreau has shown that 
the Physoclist Caranx trachurus is capable of liberating a continuous stream of air 
through its “ canal de sdretd ” when exposed to artificially diminished pressures. In 
* A few Physoclisti (e.g., Holocentrum, Friacanthus, Coesio, Ac.), in which, on the authority of Kuer 
(21), an open ductus pneumaticus is present, may also bo included. 
+ Moreau, loc. cit. 
