AMPHIBIA. 175 



MORPHOLOGY AND PHYSIOLOGY. 



1. External Characters. The bilaterally symmetrical body 

 exhibits no external trace of segmentation, and is divided into 

 head and trunk, between which no neck intervenes. There is 

 also no tail. Fore and hind limbs are present, and these, unlike 

 the paired fins of dogfish, to which they are homologous, are 

 transversely jointed, and split at their distal ends into digits. 

 The surface of the body is soft and mois", and there is no general 

 investing exoskeleton. Owing to the presence of pigment in the 

 skin the body is of a yellowish-brown, and is mottled dorsally. 

 The ventral surface is much smoother and paler than the dorsal. 

 The colour varies with the surroundings. 



The flattened head is bluntly triangular, with a forwardly- 

 directed apex. The mouth is extremely wide, and extends back- 

 wards to the posterior angles of the head. On the dorsal surface, 

 near the front and widely separated, are two small valvular aper- 

 tures, the nostrils or external nares. Behind these are the large 

 projecting eyes, with small, immobile upper eyelids, and delicate, 

 semi-transparent lower eyelids, capable of considerable movement. 



The space between the eyes is broader in JR. temporaria than in R. exc.u- 

 lenta, and while flat or convex in the former, is concave in the latter. 



Behind each eye is a circular space, the tympanic area (larger in 

 R. esculenta), which in R. temporaria is inside a dark patch of pig- 

 ment, that tapers to a point behind. In the male R. esculenta a 

 pair of vocal sacs are found, which, when inflated, appear as rounded 

 projections near the angles of the mouth. 



The trunk is somewhat oval, tapering to a blunt point behind, 

 where a small rounded cloacal aperture is found. Hard parts can 

 be felt through the skin along the entire dorsal surface, but this 

 is only the case with the anterior part of the ventral surface. In 

 this way a thoracic region in front can be distinguished from an 

 abdominal region behind. 



The fore-limb commences immediately behind the head, and is 

 divided into (1) Brachium (arm), (2) Antebrachium (fore-arm), and 

 (3) Manus (hand). 



If the fore- or hind-limb of an animal is spread out in the primitive 

 position i.e., at right angles to the body, with the "palm" or "sole" side 



