The accessory respiratory organs of Anabas testudineus (Bloch) (Anabantidae, Pisces)

Abstract

SUMMARY 1The accessory respiratory organs of Anabas testudineus consists of the following structures: (i) the supra-branchial chamber, (ii) the labyrinthine organ, and (iii) the respiratory membrane. 2The incomplete division of the supra-branchial chamber into outer and inner recesses, the structure and action of the shutter, the manner of communication between the two recesses and communication with the pharynx and opercular chamber by means of inhalent and exhalent apertures are described. 3The labyrinthine organ, made up of several saucer-shaped plates, is composed of and derived from modified, abbreviated and fused primary and secondary gill lamellae. The surface of the labyrinthine organ is divided into respiratory ‘islets’, and injected preparations have disclosed the presence of capillaries resembling those of a typical gill lamella. 4The respiratory membrane lining the supra-branchial chamber and the labyrinthine organ consists of vascular and non-vascular areas. The vascular part of the membrane comprises small and large ‘islets’ distributed over the surface. The presence of pilaster cells and the specialized character of the blood capillaries strongly suggests that the ‘islets’ are derived from the histological elements of the gill lamellae. The islets make intriguing configurations on account of the fusion of neighbouring primary gill lamellae and the flattening of the secondary lamellae. Associated with each islet are two efferent and one median afferent vessel, and several transverse channels interconnecting them. The non-vascular part of the membrane is represented by the irregular ‘lanes’ present between the islets. This represents the interlamellar areas between contiguous gill lamellae. 5The branchial, hyoid and mandibular muscles have become modified in connexion with the change-over from aquatic to aerial respiration. The adductor arcus palatini, a muscle of the hyoid segment, is modified to form the contractile floor of the anterior part of the respiratory air chamber. The branchial muscles associated with the first four gill arches and the respiratory sac have been studied and a list of them is given.