216 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
contraction of its muscle or returns to its previous position through the elastic recoil 
of its root on the muscle ceasing to contract. 
The air-bladder (figs. 91 and 92) consists of an anterior portion, which is broad in 
front and oval behind, and corresponds to the whole of a normal bladder, and also of 
a contracted tubular part extending backwards in the substance of the kidney to 
within about half-an-inch of the anus. The anterior portion (fig. 91) is divided 
internally into a short but broad anterior chamber (a.c.), and two relatively large 
lateral compartments {l.c.) by the usual primary transverse {t.s.) and longitudinal {l.s.) 
septa. The posterior section of the bladder is a simple tubular caecum com- 
municating anteriorly with both the lateral chambers but terminating blindly behind. 
The walls of the caecum are strengthened internally by a series of projecting annular 
ridges, but the longitudinal septum separating the two lateral compartments anteriorly 
extends into the cavity of the caecum for only a very short distance. The cavities of 
the two lateral chamibers (fig. 91, l.c.) are subdivided and greatly broken up by the 
formation of numerous secondary transverse septa (<.s.^), which grow out from the 
sides of the longitudinal septum and terminate laterally in free concave margins 
before quite reaching the outer walls of the chambers. The secondary septa are 
further complicated, and the spaces they enclose additionally sacculated by the 
growth of numerous root-like bundles of fibres, which extend between the opposed 
faces of the septa, and from thence into the adjacent ventral wall of the bladder. 
The primary transverse septum is unusually thick, and the width of its ventral 
margin but a little less than that of the bladder itself (figs. 91 and 92, t.s.). The 
more contracted dorsal edge of the septum is firmly adherent to the ventral surface 
and sides of the posterior portion of the complex centrum, and anteriorly to this, on 
each side, is inserted also into the posterior margin of the dorsal lamina, while the 
more lateral portions of the septum have their dorsal margins attached to the ventral 
surfaces of the posterior divisions of the transverse processes of the fourth vertebra. 
In the general structure of the walls of the anterior chamber and in the disposition 
and skeletal attachments of their component fibres Pa7igasius closely agrees with 
Macrones (fig. 92, a.c.). A noteworthy feature, however, is the thinness and trans- 
parency of its walls over a well defined oval area in each of its antero-lateral 
regions. The thinness of these areas is due to the extreme tenuity of the tunica 
externa, each area being mainly formed by the delicate tunica interna, which here, as 
in other Siluridae, lines the whole of the interior of the air-bladder, including the 
camerated lateral compartments and the tubular caecal appendage. To the external 
faces of these areas, but without being otherwise than feebly attached thereto by a 
delicate connective tissue, the oval plates of the “ elastic-spring ” mechanism are 
applied. The lateral walls of the anterior chamber are closely related to lateral 
cutaneous areas (fig. 91, Z.c.a.), but a thin stratum of fat prevents the two structures 
from coming into actual contact except perhaps for a very limited extent. 
A fairly strong transverse membrane invests the median portion of the anterior 
