ON THE ANAT05iIY OF FISHES. 
311 
development of special bony laminae (ventral processes) by the extension of bony 
deposit from the superficial ossifications investing the complex centrum with the 
superficial fibrous tissue which lies between the peritoneum and the ventral surface of 
the bladder. In such cases as these it may be conjectured tliat the retention of even 
a rudimentary air-bladder would be harmful to its owner if it were even imperfectly 
in physiological connection with the auditory organ tlirough the Weberian ossicles, 
for even a purely “ ground ” Fish may be exposed to variations of hydrostatic 
pressure in the course of its migrations at different depths, or over the uneven bed of 
a river, or, as might be the case in some instances, where the Fish inhabited tidal 
rivers, by the rise or fall of the tide, and such variations might lead to the trans- 
mission of stimuli to the auditory organ, but no responsive adjustment on the part of 
the Fish could take place on account of the utter uselessness of the air-bladder for its 
normal hydrostatic function. Physiologically, the enclosure of the reduced air- 
hladder within a bony capsule would have the effect of preventing its expansion 
under the influence of diminished pressure, and conceivably therefore subserve the 
purpose of preventing the transmission of useless and disturbing stimuli to the central 
nervous system. 
On the other hand, in several abnormal Siluroids {e.g., Bagarius, Ahjsis, Acro- 
chordonichtliys, Glyptosternum, Euclyptosteraum, &c.) the air-bladder has so far 
undergone retrogressive modification in one way or another as to have been struc- 
turally incapable of responding to variations of external pressure, or of initiating any 
movements in the chain of Weberian ossicles. In all these Siluroids the atrophy of 
the fibres forming the dorsal walls of the two lateral air-sacs constitutes a structural 
lesion so obvious that it is impossible that any alterations in the volume of the air- 
bladder could be converted into corresponding movements of the Weberian mechanism 
or in any way affect the auditory organ . In such instances complete encapsulation is, 
ex hypothesi, unnecessary, and therefore does not take place. As suggested above, 
the partial enclosure of the air-bladder within mere grooves or recesses on the ventral 
surfaces of the modified transverse processes may here be regarded as simpl}- due to 
the fact that, as the organ lost its lateral compartments and the anterior chamber 
gradually became smaller, the transverse processes still continued to retain their 
normal relations to the latter, while at the same time contracting and more or less 
curving round it. On the other hand, there are one or two abnormal forms {e.g., 
Pinielodus pulcher and Schilhichthys) in which encapsulation is but partial, although 
the structural integrity of the air-bladder is so far maintained that it might still be 
susceptible to slight volumetric changes when exposed to ])ressure variations, and also 
he able to convert such chaiiires of volume into movements of the Weberian ossicles, 
but in these, and in one or two other similar cases, it will generally be found that 
the bladder is so far environed by bone that no lateral distension can take place. 
Thus in the types referred to the osseous grooves are contracted distally in such a. 
way as to effectually check all tendency to lateral expansion on the j>art of the air- 
