236 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
posterior ca3ca is very rare {A^ichenipterus ohscurus). Very generally the longi- 
tudinal septum extends backwards into the posterior caecum, and subdivides its 
cavity into two distinct lateral canals or chambers, which communicate anteriorly 
with the proper lateral compartments of the bladder. Not infrequently the single or 
double cavity of the caecum is partially subdivided internally by a series of circularly 
disposed, inwardly projecting ridges {e.c/., Malapferurus). In some Siluroids [Pan- 
gasivs), w'here the lateral chambers are largely occupied by a trabecular network of 
fibrous bundles, the cavity of the postei'ior caecum is largely obliterated by the 
formation of a similar growth. It may be remarked that caecal appendages are very 
characteristic of those Siluroids in which an “ elastic-spring ” apparatus is present. 
They may be very rudimentary {e.g., Auchenipterus ohscurus and Oxydoras), but it 
rarely hap])ens {ep)., Auchenipterus nodosus) that they are completely absent. Even 
w'hen present and well developed in such Siluroids, there is considerable variety in 
the extent to which the cavities of these appendages are sub-divided, or even 
partially obliterated by the growth of internal septa and fibrous trabeculae. 
Except, perhaps, in the case of Rita, the significance of these caecal structures is not 
always obvious. This is more particularly the case with the posterior caecal pro- 
longations of the air-bladder, but with the antero-lateral caeca it is somewhat 
different, and a possible explanation of their existence may be suggested. 
A branching condition of the air-bladder, with the branches ending in caecal 
extremities, is very common in certain Physoclist genera,*' and occasionally such 
branches may acquire a close relation with the internal ear. In the genera Holo- 
centrum and Sargus, two caecal processes are given off from the anterior end of the 
air-bladder, which diverge as they pass forwards, and ultimately become applied 
to the fibrous membranes, closing what would otherwise be vacuities in the outer 
walls of the auditory capsules. A similar arrangement has been described by 
Parker (31) as existing in the Red Cod [Lotella hacchus), while Weber (39) first 
lecorded the existence of an analogous but more intimate connection between the 
two structures in the Herring (Clupea harengus). It may at least be conjectured 
that the connection betAveen the air-bladder and the internal ear in these Fishes 
subserves a function similar to that of the Weberian mechanism in the Ostario- 
physese, although, no doubt, in a very primitive and rudimentary fashion, and, 
therefore, tl)at these caecal structures represent an initial stage in the evolution of 
that mechanism. We are not aware of the existence of antero-lateral caeca in any 
Ostariophyseae but the Siluridae. Sagemehl (33), however, has described the tunica 
externa of the air-bladder in certain Characinidae, as being prolonged forwards in 
the form of a twisted ligament, which anteriorly splits into two, and is attached 
to the posterior portion of the base of the cranium. Although these ligaments are 
solid, and have no lumen, and it is expressly stated that the tunica externa has no 
share in their formation, Sagemehl regards them as the atavistic vestiges of 
* See GOnther (14, pp. 141-145 ; also 15). 
