EVALUATION OF CAUSES IN RISE OP AMPHIBIANS 423 



the area of respiratory surface is considerably increased by the development 

 of folds and processes of the intestinal mucous membrane. 



"In a few tropical Teleosts curious labyrinthiform organs are developed in 

 connection with certain of the branchial arches, and serve as accessory breath- 

 ing organs. In the Indian 'Climbing Perch' (Anabas scandens), of the family 

 Anabantidae, the organ consists of three or more concentrically arranged bony 

 lamina?, with wavy, crenulated margins, attached by a common bony base to 

 the upper extremity of the fourth branchial arch, and enclosed in a special 

 dorsal enlargement of the branchial cavity. The vascular membrane which 

 invests the laminae is abundantly supplied with venous blood by a branch of 

 the fourth afferent branchial artery, the equivalent efferent vessel joining the 

 dorsal aorta. Essentially similar organs are found in several genera of Os- 

 phromenidse (e. g., Poly acanthus, Osphromenus, and Triehogaster) . A simpler 

 form of respiratory organ of somewhat the same type occurs in the Indian 

 family Ophiocephalidae. In these Fishes there is, on each side, an accessory 

 branchial cavity, situated above that which contains the gills, but freely com- 

 municating with it. The cavity is lined by a thickened and puckered vascular 

 membrane, but otherwise contains no special respiratory structures. 



"In the Siluroid genera Clarias and Heterobranchus the accessory organ 

 takes the form of branched, arborescent and highly vascular structures, de 

 veloped as outgrowths from the dorsal extremities of one or two branchial 

 arches, and enclosed within a posterior and dorsal expansion of the proper 

 branchial cavity. 



"Another example of these interesting structures occurs in Chanos salmoneus 

 and a few other Clupeidae in the shape of a coiled gill-like organ ('gill-helix'), 

 which is supported by the dorsal segment of the fourth branchial arch, and 

 enclosed in a similarly curved caeeal extension of the branchial cavity. Each 

 'gill-helix' derives its blood from the fourth afferent branchial artery, the cor- 

 responding efferent vessel joining the fourth efferent branchial artery. A sim- 

 ilar spirally coiled 'gill-helix' is found also in Heterotis ehrenbergii, amongst 

 the Osteoglossidae, and in several species of Characinidae. 



"In other Teleosts the accessory breathing organ assumes the condition of 

 paired lung-like outgrowths of the branchial cavity. Thus, in one of the Sym- 

 branchidse, the Indian 'Cuchia Eel' (Amphipnous cuchia), there is a pair of 

 small bladder-like sacs, with membranous and vascular walls, each of which 

 opens into the branchial cavity above the first gill-cleft, and is supplied with 

 blood by the afferent branchial artery of the gill-less first branchial arch. An 

 extreme modification in the same direction is presented by the Indian Siluroid 

 Saccobranchus. In this Fish a long caeeal diverticulum of the branchial cavity 

 extends backwards on each side from the dorsal region of the first branchial 

 cleft to the tail, and in its course is situated internally to the lateral trunk 

 musculature, and close to the vertebral column. The walls of the caeca are 

 vascular, but no special respiratory structures are developed within their cavi- 

 ties, which, during life, only contain air. In 8. singio the right caecum is sup- 

 plied with blood by an extension backwards of the dorsal portion of the first 

 afferent branchial artery of that side ; the left, on the contrary, being supplied 

 by the corresponding portion of the fourth afferent artery of the same side. 

 In S. fossilis both air-sacs are supplied by the fourth afferent branchial artery. 

 The efferent vessels join the fourth efferent branchial artery, right or left, as 

 the case may be. 



