324 THE DOG-FISH LESS. 



THE Doc-FiSH. 1 



A dog-fish is bilaterally symmetrical, the nearly cylin- 

 drical body (Fig. 79, A) terminating in front in a blunt 

 snout and behind passing insensibly into an upturned tail. 

 Externally there is no appearance of segmentation. 



The mouth (MtJi) is on the ventral surface of the head 

 or anterior region of the body ; it is transversely elongated, 

 and is supported by jaws which are respectively anterior 

 (upper) and posterior (lower). They thus differ funda- 

 mentally from the jaws of arthropods, which are modified 

 appendages and are therefore disposed right and left. 



A short distance behind the mouth are five vertical slits 

 (B, Ext. br. ap) arranged in a longitudinal series, the 

 external branchial* apertures or gill-clefts. The vent, or 

 cloacal aperture (An) is situated on the ventral surface a 

 considerable distance from the end of the tail. That part 

 of the body lying in front of the last gill-cleft is counted as 

 the head, all behind the vent as the tail, the intermediate 

 portion as the trunk. 



Appendages are present, but in a very different form from 

 those of the crayfish. They consist of flat processes of the 

 body-wall called fins. Two of them (D.F^,D. F*) are 

 situated in the middle line of the back (dorsal fins) : one 

 (V.F) in the middle ventral line behind the cloacal aperture 

 (ventral fin\ and one (C.F) is attached to the up-turned end 

 of the tail (caudal fin) : all these being unpaired structures or 

 median fins. Then there is a pair of pectoral fins situated 



1 For a detailed description of a dog-fish see Marshall and Hurst, 

 Practical Zoology (London, 1892), p. 206. For descriptions of other 

 fishes, equally suitable in some respects as types of Vertebrata, see 

 Rolleston and Jackson, Forms of Animal Life (Oxford, 1888), pp. 83 

 and 273 : and Parker, Zoototny (London, 1884), pp. I, 27, 86. 



