64 ZOOLOGY SEOT. 



through the whole group, both as to the general arrangement of 

 the various systems of organs and the structure of the organs 

 themselves far greater than in any of the principal invertebrate 

 groups. The range of variation in the whole of the six classes 

 included in the division is, in fact, considerably less than in many 

 single classes of Invertebrata for instance, Hydrozoa or Crustacea. 

 Hence, while the plan hitherto adopted of treating the group class 

 by class will be followed, it will be found convenient to begin by 

 devoting a considerable space to a preliminary account of the 

 Craniata as a whole, since in this way much needless repetition 

 will be avoided. 



The Craniata include the following classes and sub-classes : 



CLASS I. CYCLOSTOMI, 

 Including the Lampreys and Hags. 



CLASS II. PISCES, 

 True Fishes, which are again divisible into 



Sub-class 1. Elasmobranchii, 

 Including the Sharks and Rays. 



Sub-class 2. Holocephali, 



Including the Cat-fish (Chimcera) and the Elephant-fish 

 (Callorhynchus). 



Sub-class 3. Tekostomi, 



Including the bony Fishes, such as Perch, Cod, Trout, &c., and the 

 Sturgeons and their allies. 



Sub-class 4. Dipnoi, 1 

 Mud-Fishes. 



CLASS III. AMPHIBIA, 

 Including Frogs, Toads, Newts, and Salamanders. 



CLASS IV. REPTILIA, 

 Including Lizards. Snakes, Crocodiles, Turtles, and Tortoises. 



CLASS V. AVES, 

 Birds. 



i The animals included in Classes I. and II. are all "Fishes " in the broad sense of the word. 



