xii PHYLUM CHORDATA 329 



roughness is found to be due to the presence of innumerable 

 minute hard granules, set closely together so as to give the 

 surface the character of a fine file. The general shape of 

 the body is adapted to cleaving the water rapidly long 

 and narrow, nearly fusiform,, pointed at the ends and the 

 fins are obviously swimming organs. The fins are all of 

 the same general character, so far as their superficial ap- 

 pearance is concerned : they are all of the nature of flap- 

 like outgrowths, thick at the base, where they are obviously 

 supported by hard parts, and thinner towards the margins, 

 where their sole support is a series of slender fibres of 

 horny character. Besides the two pairs of fins which have 

 already been referred to as representing the anterior and 

 posterior pairs of limbs in the Lizard and the 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 (df. I, df. 2), the single ventral 

 (v.f.\ and the single caudal (cd. f.\ the last fringing the tail. 



Behind the eye in the Dog-fish 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, the spiracle (sp.\ does not however lead into the 

 ear, but into the cavity of the pharynx. Further back there 

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

 are the branchial or gill-clefts (ex. br. ap.\ 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 rhythmically taken in through the mouth 

 and expelled by the spiracles and branchial clefts : these 

 are the movements of respiration. 



The mouth is situated some little distance behind the 



