448 ZOOLOGY. 



class of Vertebrata under the name Cyclostomata. The 

 condition of the mouth and pituitary sac is so extraordinary 

 and so different from the usual condition which, with 

 modifications, is common to all other Vertebrates that 

 modern zoologists have given the Cyclostomes a still more 

 isolated position and divided the Vertebrata into two main 

 divisions, one, the Cyclostomata, containing only these 

 peculiar forms, and the other, the Gnathostomata, containing 

 the rest of the Vertebrates. Lately it has been stated as 

 the result of new investigations that the so-called tongue 

 of the Myxinoids is really a modified mandibular arch, and 

 this would suggest that the Cyclostomes are modified 

 fishes ; but the position of the pituitary sac is still so 

 exceptional that we cannot unite them with the latter, but 

 must continue to regard them as a separate class. 



CLASS : PISCES. 



Craniata which possess gill- slits and gills throughout 

 life, and which possess paired fins or ichthyopterygia, 

 and usually unpaired fins also. This character distinguishes 

 all fishes from Amphibia, for although the fins of fishes 

 differ from one another in structure they never resemble 

 the limb of the terrestrial Vertebrate consisting of a 

 segmented axis terminating in five digits. 



The fishes consist of the following orders : 



Order 1, Elasmobranchii. G-ill slits opening on surface 

 of body, not covered by an operculum. Skeleton cartila- 

 ginous, more or less calcified, but not ossified. Exoskeleton 

 consisting of dermal denticles. Skull hyostylic, i.e. jaws 

 suspended by the hyomandibular cartilage. A preoral 

 rostrum or snout. No air-bladder. Truncus arteriosus 

 with several rows of valces. Fins with long cartilaginous 

 endoskeletal rays, and beyond these dermal horny fibres. 



Order 2, Holocephali. Similar to Elasmobranchs, but 

 differing in the following characters : dermal denticles 

 absent except on the frontal clasper ; bony rings surround- 

 ing the notochord ; skull autostylic, i.e. the palato-quadrate 

 is fused with the cranium and the jaws are not suspended 

 by the hyomandibular ; gill-slits covered by an operculum. 



