294 ZOOLOGY SECT. 



Systematic Position of the Example. 



The genus Rana belongs to the family Ranidce, which with 

 three other families constitutes the series Firmisternia, of the 

 sub-order Phaneroglossa and order Anura. 



The absence of a tail and the presence of two pairs of limbs, of 

 which the posterior are larger than the anterior, place the genus 

 among the Anura. The presence of a tongue and of distinct 

 paired Eustachian tubes separates the Phaneroglossa from the 

 Aglossa (Pipa and Xenopus), a small group of Toads in which the 

 tongue is absent and the Eustachian tubes have a common median 

 opening. The Firmisternia are distinguished by having the 

 coracoids joined by a common epicoracoid cartilage in contra- 

 distinction to the Arcifera (Tree-frogs, Toads, &c.), in which the 

 epicoracoids overlap one another. The Ranidaa are distinguished 

 from the other families of Firmisternia by having teeth in the 

 upper jaw and the transverse processes of the sacral vertebrae not 

 dilated. R. temporaria is distinguished from R. esculenta by its 

 smaller size and brown colour, by the large black patch in the 

 tympanic region, and by the absence of external vocal sacs in the 

 male. 



3. GENERAL ORGANISATION. 



The Amphibia are specially interesting as illustrating the 

 transition from the water-breathing to the air-breathing type of 

 Craniate structure. The lower forms retain their gills throughout 

 life, but possess lungs in addition : in the higher the gills occur 

 only in the larval state, and the adult breathes exclusively by the 

 lungs and skin, becoming transformed from an aquatic into a 

 terrestrial animal. At the same time further adaptations to land- 

 life take place, the most important being the modification of the 

 blood-vessels consequent'on the disappearance of the gills, the loss 

 of median fins, and the strengthening of the limbs to support 

 the weight of the body. 



External Characters. An excellent example of the lower 

 Urodela with persistent gills is afforded by the great North Ameri- 

 can Water-newt, Necturus maculatus (Fig. 941). The animal 

 attains a length of 30 cm. (more than a foot) ; the elongated trunk 

 is separated by a slight constriction from the depressed head, and 

 passes insensibly into the compressed tail, which is bordered by a 

 continuous median fin unsupported by fin-rays. The limbs are 

 small and weak in proportion to the size of the body, and in 

 the ordinary swimming attitude are directed backwards, more or 

 less parallel to the sagittal plane, the upper arm and thigh 

 taking a direction backwards and slightly upwards, the fore-arm 

 and hand and the shank and foot extending backwards and 

 downwards. Each limb thus presents an external or dorsal arid 



