292 ZOOLOGY 



SECT, 



The Jungs now appear', and the larva is for a time truly 

 amphibious, rising periodically to the surface to breathe air : the 

 single branchial aperture, however, soon closes, and henceforth 

 respiration is purely aerial. 



In the meantime the limbs are developed. The hind-limbs 

 appear as little rounded buds, one on each side of the root of the 

 tail (5). The fore-limbs arise beneath the operculum and are 

 therefore hidden at first ; soon, however, they emerge by forcing 

 their way through the operculum. As the limbs increase in size 

 the tail undergoes a progressive shrinking (6-8} . The mouth 

 widens by the backward rotation of the suspensorium, the in- 

 testine undergoes a relative diminution in length, and vegetable is 

 exchanged for animal diet. The little, tailed Frog can now leave 

 the water and hop about upon land ; its tail is soon completely 

 absorbed, and the metamorphosis is complete. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Amphibia are Craniata which, in nearly all cases, possess 

 gills either in the larval state only or throughout life, and which 

 usually breathe by lungs in the adult condition. The skin is 

 glandular, and there may or may not be a bony dermal exoskeleton. 

 When unpaired fins are present, they are never supported by fin- 

 rays. The paired appendages, when present, are pentadactyle limbs : 

 the digits are usually devoid of claws. The skull is autostylic and 

 is articulated with the first vertebra by paired occipital condyles 

 borne on the exoccipitals. The basioccipital arid supraoccipital 

 are usually, and the basisphenoid is always, absent: there is a 

 large parasphenoid and there are " well-developed paraquadrates 

 (squamosals). In the branchiate forms large hyoid and branchial 

 arches persist throughout life : in the non-branchiate species these 

 structures undergo more or less degeneration and give rise to the 

 hyoid-cartilage. The heart has a sinus venosus, right t and left 

 auricles, a single ventricle, and aconus arteriosus; the aortic arches 

 arise from a bulbus aortas or abbreviated ventral aorta. The 

 cardinal veins undergo more or less degeneration and are practically 

 replaced by an unpaired postcaval vein. There is a renal portal 

 system, part of the returning blood from the posterior parts of the 

 body going through it, the rest through the hepatic portal system by 

 an abdominal vein which represents fused lateral veins. The red 

 corpuscles are oval and nucleated and are often of unusual size. 

 The lymphatic system is well developed. In the brain the small 

 'size of the cerebellum is noticeable. The olfactory sacs open into 

 the mouth by posterior nares. The outer wall of the auditory 

 capsule is pierced by a fenestra ovalis into which is inserted a 

 cartilaginous stapes : the stapes may be connected by a columella 



