296 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
close relation of the lateral walls of the air-bladder to the external skin, and therefore 
to the surrounding medium, enables the air-bladder more promptly to react to 
pressure oscillations is, perhaps, not improbable, and, if such be the case, would 
certainly have the effect of rendering the Siluridse much more sensitive to variations 
of hydrostatic pressure than is the case with other Ostariophyseae where the air- 
bladder is separated from the external skin by the lateral lobes of the liver and other 
abdominal viscera. While admitting the feasibility of this suggestion, we may point 
out that the connection of the air-bladder with the superficial skin would also enable 
variations in the size of the aii’-bladder, the result of pressure variations, more 
promptly to modify the volume and specific gravity of the Fish, for any expansion or 
contraction of that organ from such causes must at once lead to the inward or 
outward bulging of the lateral cutaneous areas. The special advantage of lateral 
cutaneous areas to these Fishes, from this point of view, would lie in the fact that 
their greater susceptibility to alterations of volume and specific gravity would 
probably ensure a corresponding increase in the delicacy of the responsive processes 
involved in pressure adjustment. 
The possibility, however remote, that these anatomical features have no special 
nhysiological value, but are simply the necessary result of other structural modifica- 
tions of undoubted utility, must also be kept in view. The relative shortness of the 
abdominal cavity in many Siluroids may have caused the lateral expansion of the 
air-bladder and its consequent abutment against the external skin. That this may 
be the case is suggested by the practical suppression of the lateral cutaneous areas in 
some Siluridae {e.g., Malapterurus) where the abdominal cavity is of greater relative 
length than usual. One of the conditions of the formation of lateral cutaneous areas, 
viz., the dorsal deflection of the dorso-lateral trunk muscles, may be only a necessary 
consequence of the elongation and expansion of the transverse processes of the fourth 
and fifth vertebrae, and their disposition at right angles to the long axis of the 
vertebral column for the investment of the dorsal wall of the anterior division of the 
air-bladder. In the Cyprinidae, where lateral cutaneous areas are wanting, the 
transverse processes of the fourth vertebra are of considerable length without being 
specially expanded, but, like true ribs, their marked ventral flexure prevents them 
interrupting the straightforward course of the lateral trunk musculature, or pro- 
ducing any upward deflection of the dorso-lateral muscles. If this alternative view 
is at all a reasonable one, it will follow that the existence of the areas will primarily 
depend on the investment of the dorsal surface of the anterior chamber by the 
expanded transverse processes, which in turn must be regarded as a contrivance for 
preventing the dorsal expansion of the chamber during pressure variations, thereby 
restricting such alterations of volume to movements of the lateral walls alone. 
{h.) The “ Elastic-spring " Apparatus . — In the great majority of the Ostariophyseae 
the escape of air from the air-bladder through the ductus pneumaticus apparently 
takes place only as the result of the expansion of the contained gases under the 
