72 Miscellaneous. 



distinction may be possible between the first two when the class of 

 Fishes has the wide range generally allowed, there is no difficulty in 

 their discrimination with the limits here to be assigned to them. 

 We may then group the classes as follows : — 



Ou the one hand is Branchiostoma or Ampli'ioxus, distinguished 

 by the extension of the notochord (which is, of course, persistent) to 

 the anterior end of the vertebral column, the attenuation of the 

 spinal cord forward and its simple structure, the absence of auditory 

 organs, the simple tubular structure of the heart, and the develop- 

 ment of the liver simply as a diverticulum of the intestine. — This 

 type is called by Hiickel the subphylum Leptocardia or Acrania. 



On the other hand are all the other vertebrates, which agree in 

 the termination of the notochord behind the pituitary fossa, the en- 

 largement of the spinal cord forward into a brain, the development 

 of auditory organs, the division of the heart into (two to four) 

 chambers, which in part (one or two) specially receive the blood, 

 and in part (one or two) specially distribute it to the body again, 

 and the differentiation of the liver as an independent and highly spe- 

 cialized organ. — Tliis group is named by Hackel the subphylum 

 PacJajcnrdia or Craniota. 



Tlie numerous forms belonging to the last " subphylum " are also 

 divisible into two great groups. 



In one the skull has no cincture girdling the mouth, and conse- 

 quently no lower jaw, there are no pectoral members or scapular 

 girdle, and there is but one nasal sac, which has a median external 

 aperture. — To this section belong the lampreys and hags, the repre- 

 sentatives of the class of Marslpohranclis. 



In the other, the skull has a cincture surrounding the mouth, its 

 inferior portion being specialized as a lower jaw; they have (arche- 

 typically at least) a pectoral member and a shoulder-girdle de- 

 veloped ; and there are two nasal sacs, each having an olfactory 

 nerve distributed to an external aperture. — These vertebrates are 

 again divisible into three groups or superclasses. 



1. In the first {Lyrifera) the shoulder-girdle forms a lyriform 

 or furcula-shaped apparatus, the scapuliB and their adjuncts of 

 both sides being connected together below along the median line, 

 and an air-bladder (sometimes lung-like) is typically developed 

 (sometimes, however, atrophied) and (1) either connects with the 

 oesophagus by a single duct or (2) is entirely closed. To this 

 superclass belong the classes of Fishes and Elasmobranchiates. 



2. In the second (Quadrafifcra) the shoulder-girdle is repre- 

 sented by the scapuljE and their appendages, which are limited 

 to the respective sides, a sternum is differentiated, and instead 

 of an air-bladder are two lungs, each with a special canal, which 

 communicate with the pharynx. The lower jaw is compound and 

 is articulated with the skull by the intervention of a special bone — 

 the OS quadratum. To this superclass belong the Batrachians, the 

 lleptiles, and the Birds, the last two forming the group Sauropsida. 



3. In the third (M(dleifcra) tlic siioulder-girdle is represented 

 by composite scapulae, limited to the sides or back ; a sternum is 



