18S9.J Contributions to the Anatomy of Fishes. 



319 



development of two large lateral caeca from the anterior chamber. 

 Apart from its partial longitudinal construction into two laterally 

 bulging halves, — a separation which in some cases may be emphasised 

 by the formation of one or two longitudinally arranged and inwardly 

 projecting ridge-like aggregation of fibres from the median line of 

 the posterior, ventral, and anterior walls, — the cavity of the anterior 

 chamber has smooth walls, and is not subdivided by the growth of 

 internal septa. 



The lateral compartments may also have undivided cavities 

 (Auchenaspis, Callichrous, and 8 Hums), but not infrequently they are 

 rendered more inexpansible, and possibly at the same time less 

 compressible, by the formation of a variable number of secondary 

 transverse septa (e.g., Macrones), which incompletely subdivide each 

 chamber into a series of transversely arranged, intercommunicating 

 spaces. Occasionally the excessive development of these septa and 

 their union by root-like bundles of fibres, which pass between their 

 opposed surfaces, may lead to the formation of a thick trabecular 

 network of fibrous columns or bands, and to the partial obliteration of 

 the cavities of the two chambers (Pangasius). 



The width of the primary transverse septum forming the posterior 

 wall of the anterior chamber varies greatly in different Siluroids. In 

 some (e.g., Auchenaspis) the septum is co-extensive with the width of 

 the air-bladder, although contracted dorsally to admit of the lateral 

 chambers communicating with the anterior chamber ; in others 

 (Callichrous, Cryptopterus) the septum is reduced to the condition of 

 a narrow, but stout, column-like aggregation of fibres. 



Caecal appendages to the anterior and lateral compartments are 

 not uncommon. The anterior chamber may have small anterior caeca 

 (Macrones aor), or much smaller antero-lateral caeca (Osteogeniosus). 

 Lateral caeca are sometimes present, and may either take the form of 

 large funnel-shaped structures, which extend the whole length of the 

 abdominal cavity and are entirely free from internal subdivisions 

 (Rita), or may occur as small forwardly directed outgrowths, sub- 

 divided internally by a network of fibrous bundles, and communicating 

 with the anterior chamber by a number of slit-like orifices in its 

 lateral walls (Platy stoma) . Very rarely (Gallophysis) are the lateral 

 caeca so numerous as to form a wreath round the lateral regions of 

 the chamber. The lateral compartments are frequently either con- 

 stricted or prolonged into a posterior caecal appendage. This may be 

 a longer or shorter tubular, or a slightly oval structure, and confined 

 to the abdominal cavity (Pangasius buchanani, Bagroides melanopterus), 

 or a long, tapering, tubular structure, which, after traversing the 

 abdomen, extends for some distance along the right side of the tail, 

 between the haemal arches and the lateral musculature (Cryptopterus 

 micronema). In some cases the posterior caecum is very large, and in 



VOL. xlvi. 2 A 



