AMPHIBIA. 205 



these organs, and the contraction of the muscle-fibres they contain. 

 The contraction of the abdominal walls may also assist. 



The thin-walled lung-capillaries are only separated from the 

 air contained in the lung-cavities by a layer of epithelium, and, 

 therefore, carbon dioxide can readily diffuse out of, and oxygen 

 into, the blood which they contain. A similar interchange takes 

 place between the blood in the capillaries of the skin and the 

 oxygen in the surrounding air or water. This cannot take place 

 in air unless the skin is damp. 



The haemoglobin of the red corpuscles plays the usual important 

 part in respiration. 



Voice is produced by the vocal chords, the edges of which can 

 be brought parallel to one another by certain muscles, and then 

 thrown into vibration by the expired air. The vocal sacs in the 

 male, R. esculenta, serve as resonators. 



7. Urinogenital Organs, including excretory and reproductive 

 organs. 



The excretory organs (Figs. 51 and 60) are two elongated,. 

 flattened, reddish-brown kidneys, symmetrically disposed in the 

 posterior part of the subvertebral lymph-sinus, and covered 

 with pleuro-peritoneum on their ventral surfaces only. A 

 slender tube, urinary duct in the female, urinogenital duct in the 

 male, runs from the outer side of each kidney to open into the- 

 dorsal side of the cloaca, close to its fellow, by a minute slit. 

 Directly opposite this there is a rounded aperture in the ventral 

 cloacal wall which leads into the urinary bladder, a large bilobed 

 sac with delicate membranous walls. 



Closely connected with the excretory organs are two anomalous 

 tures, the adrenals and fat-bodies. The former are two narrow bodies, of 

 yellowish colour, one of which is closely imbedded in the ventral surface 

 of each kidney. The fat-body (corpus adiposum) is a bright, orange- 

 coloured tuft of finger-like processes attached to the front end of the 

 kidney of either side. It consists of a network of connective tissue, ia 

 the meshes of which numerous fat-cells are imbedded. These are spherical 

 bodies full of fat, and covered by a thin layer of protoplasm with a nucleus 

 on one side. They result from the metamorphosis of ordinary connective- 

 tissue cells. 



The kidney is mostly made up of a large number of uriniferow 

 tubules. Each of these commences in a thin-walled Bowman's 

 capsule, lined by delicate squamous epithelium, into which has 

 been pushed from the outside, so to speak, a tuft of capillaries-,. 



