1889.] 



Contributions to the Anatomy of Fishes. 



313 



The air-bladder varies greatly in degree of development, not only 

 in different genera, but in different species of the same genus. Even 

 individual variations are not infrequent. Very rarely does it exhibit 

 the bipartite division into an anterior and a posterior sac so charac- 

 teristic of other families of Ostariophyseee. One of its most note- 

 worthy features is a tendency to lateral development, whereby the 

 outer walls of the anterior portion become applied, through the 

 divergence of the dorso-lateral and ventro-lateral muscles of the body 

 wall, directly to the external skin ("lateral cutaneous areas "). The 

 insertion of the crescentic processes of the tripodes is always into the 

 dorsal wall of the anterior chamber of the air-bladder in the normal 

 Siluridee, or into the corresponding walls of the laterally situated air- 

 sacs in the abnormal forms, and takes place in such a way that the 

 fibres forming the anterior and lateral walls of each half of an anterior 

 chamber, or of each air-sac, converge as they pass into and form the 

 dorsal wall, and ultimately become inserted into the convex outer 

 margin of the tripus of that side. Specialised fibres of the dorsal wall 

 ("radial fibres ") converge like the radii of a circle from the inner 

 concave margin of the crescentic process, and are inserted either 

 directly into the adjacent lateral surface of the complex centrum or 

 indirectly through the intervention of an osseous nodule ("radial 

 nodule "). Except, perhaps, in cases where the same effect is produced 

 by their partial encapsulation by bone, the anterior chamber of the 

 bladder, and its equivalent the laterally- situated air-sacs of the 

 abnormal Siluridee, generally have their walls so attached to, or 

 buttressed by, rigid portions of the axial skeleton, that only their 

 outer or lateral walls are capable by inward or outward bulging of 

 allowing variations in the internal capacity of the bladder to take 

 place. 



A ductus pneumaticus is very generally, but not invariably, 

 present. 



In nearly all Siluroids the lateral growth of the air-bladder, and 

 the intimate relation of its outer walls to lateral cutaneous areas, have 

 led to the displacement of the lateral lobes of the liver and their 

 enclosure within peritoneal cul-de-sacs — a condition which usually 

 persists even in cases where the air-bladder has undergone partial 

 atrophy. 



In many of the features to which reference has just been made, the 

 Siluridee differ from all the other families in which a Weberian 

 mechanism is present. 



As a convenient means of summarising the more important generic 

 and specific variations, the Siluroids may be somewhat arbitrarily 

 divided into two principal groups : — (I) the Siluridce normales, and 

 (II) the Siluridce abnormales. In the former group the air-bladder 

 is always well developed and subdivided internally into three inter- 



