332 MANUAL OF ZOOLOGY sect. 



support is a series of slender fibres of horny character. 

 Besides the two pairs of fins which have already been re- 

 ferred to as taking the places of the anterior and posterior 

 pairs of limbs in the lizard and rabbit, certain others are to 

 be recognised which are of a totally different character, 

 being median or unpaired ; these, which are not in any 

 way represented in either the lizard or the rabbit, are the 

 two dorsal, the single ventral, and the single caudal, the 

 last fringing the tail. 



Behind the eye in the dogfish will be noticed a small 

 aperture which seems to occupy very nearly the position 

 occupied by the opening of the ear in the rabbit. This 

 opening, however, the spiracle, does not lead into the ear, 

 but into the cavity of the pharynx. Further back- there 

 are, on each side, five slit-like apertures in a row : these 

 are the branchial or gill-clefts, and are not present in the 

 lizard or the rabbit. In the living fish it will be observed 

 that there are regular movements of the mouth, spiracles, 

 and branchial clefts, indicating that water is being rhythmi- 

 cally taken in through the mouth and expelled by the 

 spiracles and branchial clefts. Those are the movements 

 of respiration. 



The mouth is situated some little distance behind the 

 anterior extremity of the head, on the ventral side. In 

 front of it are the nasal openings (nostrils), which are also 

 ventrally situated. 



In the lizard the surface is covered with a system of 

 overlapping horny scales. The head is separated from the 

 trunk by a distinct constricted region, the neck. The tail 

 is extremely long and narrow. The two pairs of limbs, 

 anterior and posterior, ox pectoral and pelvic, are adapted to 

 running on the surface of the ground. Each consists of 

 three divisions, — arm, fore-arm, and hand, — the anterior 



