xiii PHYLUM CHORDATA 59 



other words, Vertebrata having a skull, a highly complex brain, a 

 heart of three or four chambers, and red blood-corpuscles. 



In spite of the obvious and striking diversity of organisation 

 obtaining among Craniata, between, for instance, a Lamprey, a 

 Pigeon, and a Dog, there is a fundamental unity of plan running 

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

 the various systems of organs and the structure of the organs them- 

 selves, 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. CYCLOSTOMATA, 

 Including the Lampreys and Hags. 



CLASS II. PISCES, 

 Including the true Fishes, which are again divisible into 



Sub-class 1. Elasmobranchii, 

 Including the Sharks and Rays. 



Sub-class 2. Holocephali, 



Including only the Cat-fish (Ghimcera) and the Elephant-fish 



(Callorhynchus). 



Sub-class 3. Teleostomi, 



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

 Sturgeons and their allies. 



Sub-Class 4. Dipnoi?- 

 Including the Amphibious Fishes or Mud-fishes. 



CLASS III. AMPHIBIA, 

 Including Frogs, Toads, Newts, and Salamanders. 



1 The animals included in Classes I and II are all "Fishes" in the broad 

 sense of the word 



