14 A Guide to the Zoological Collections 



(3) A digestive tube of which the fore end is in part 

 modified for breathing purposes. 



In addition, almost all Vertebrata are bilaterally 

 symmetrical, and most have a heart, placed towards 

 the ventral side of the body, and a closed system of 

 vessels in which red blood circulates : also in a great 

 majority there are two pairs of limbs suspended from 

 the vertebral column. 



The Vertebrata are arranged by Professor Ray 

 Lankester in four branches, namely, Craniata, Ccphalo- 

 chorda (Amphioxus), Urochorda (Tunicata), and Hemi- 

 chorda (Balanoglossus). 



The three last have been already treated, so that we 

 are now concerned only with the Craniata. 



The Craniate Vertebrates are divided by the same 

 authority into two " Grades," namely, Cyclostoma or 

 jawless Craniates, and Gnathostoma or Craniates with 

 jaws and with (persistent or transient) branchial bars. 

 The Cyclostomes have just been spoken of, so that we 

 now are left with the Gnathostome Craniates alone. 

 These are grouped in five great Classes : — 

 Fishes, Amphibia, Reptiles, Birds and Mammals. 

 All the members of these five classes possess jaws 

 and, at some time of life, a series of bars, known as 

 branchial arches, suspended behind the jaws. 



In the Reptiles, Birds and Mammals, the branchial 

 arches, except the first which becomes the hyoid bone, 

 disappear as independent structures early in embryonic 

 life, as they do in adult life in many Amphibia : but in 

 the Fishes, as well as in most Amphibia during larval 

 life at least, they are important structures, most of them 

 bearing gills for breathing air dissolved in water. 



The Fishes and Amphibia are thus most closely 

 related to one another, and they further agree with one 



