xii PHYLUM CHORDATA 359 



CLASS I. CYCLOSTOMI 



The lowest of existing Craniate Vertebrates are certain 

 fish-like animals known as " Lampreys " and " Hag-fishes " 

 or " Slime-fishes," which are looked upon as constituting 

 a distinct class of Craniata to which the name of Cyclos- 

 tomi is applied. Of these it is possible to make only the 

 briefest mention here. The Lampreys (Petromyzon and 

 other genera) and the Hag-fishes, or Slime-fishes (Myxine and 

 Bdellostoma) are somewhat eel-like in general shape : that is 

 to say, they have a long and narrow body without marked 

 external distinction into regions, and with a soft and slimy 

 integument. Of the fins of such a fish as the Dog-fish the 

 median or unpaired series alone are represented, paired 

 fins corresponding to the limbs of the higher Craniata being 

 entirely absent. There is a dorsal fin divided into two in 

 the Lampreys, undivided in the Hag-fishes, which is con- 

 tinued as a tail fin round the posterior or caudal extremity 

 of the body. On the lower or ventral surface of the anterior 

 or head end is a deep hollow the buccal funnel, much 

 more conspicuous in the Lampreys (Fig. 207, buc.f.}, than 

 in the Hags (Fig. 208), at the bottom of which the small 

 opening of the mouth (mth.) is situated. There are no 

 jaws, but on the inner surface of the buccal funnel and 

 on the tongue a process below the opening of the mouth 

 are a number of horny teeth (Fig. 207, 7.). In Myxine the 

 funnel is edged with slender, flexible processes or tentacles. 

 At the sides of the head are the eyes, well-developed and 

 conspicuous in the Lamprey, imperfect and buried beneath 

 the skin in Myxine, and on the upper surface is a single 

 median aperture the nostril (na. ap.). Further back at 

 the sides of the head are in the Lamprey a series of seven 

 pairs of slits, the gill-slits, leading to the gill pouches : in 





