6 VERTEBEATA. 



possession of a cellular cord, — the ' notochord,' — which runs un- 

 derneath the central nervous system, and which in the higher forms 

 is surrounded by the permanent vertebral column and skull, and is 

 largely obliterated by the development of these structures. So the 

 term Chordata is frequently employed as synonymous with Ver- 

 TEBRATA in its wide sense." (R. R. Wright.) 



Without further discussion of the Vertebrata or " Chor- 

 data" as a whole, we may proceed to the account of the several 

 subordinate groups or classes. The existing forms may first be 

 divided into about six primary groups, which have been called 

 " provinces " by Professor Huxley. These are (I) the Urochor- 

 data, including the class Tunicata ; (II) the Hemichordata or En- 

 teropneusta ; (III) the Cephalochordata, corresponding to the class 

 Leptocardii; (IV) the Ichthyopsida, including the classes of Cyclo- 

 stomi, Pisces, and Batrachia; (V) the Sauropsida, including the 

 Reptilia and Aves ; and finally (VI) the Mammalia, correspond- 

 ing to the single class of the same name. 



The relations of these provinces and classes are shown in the 

 following analysis taken, in part, from Dr. Gill's " Arrangement 

 of the Families of Fishes." Only the more obvious characters 

 are here mentioned. Others may be found in the more elaborate 

 works on Comparative Anatomy. 



Analysis of the Classes of Chordata. 



a. Anterior end of the central nervous axis not dilated into a brain, and not 

 surrounded by a protective capsule or skull. 

 4. Notochord confined to the tail and usually present only in the tadpole- 

 like larval stage of the animal (Urochordata) : adult animal not 

 fish-like nor worm-like, its body invested with a tough envelope or 



"tunic." Tunicata, A. 



66. Notochord not confined to the tail, but extending forward to the anterior 

 end of the body; sides of body with numerous gill slits which are 

 persistent through life. 

 d. Notochord developed in anterior end of body only (Hemichor- 

 data) : adult animal worm-like, without trace of fins ; a long 



proboscis before the mouth Enteropnedsta, B. 



dd. Notochord perfect, continued forward to a point before the mouth 

 (Cephalochordata) : body elongate, lanceolate, somewhat fish- 

 like in form, not worm-like nor enveloped in a "tunic"; middle 

 line of body with rudimentary fins; no proboscis; the mouth slit- 

 like, fringed with cirri . . Leptocardii, C. 



BO. Anterior end of the nervous axis dilated into a " brain," which is con- 

 tained within a protective capsule, the " skull " ; notochord not 

 continued forwards beyond the pituitary body; heart developed and 

 divided into at least two parts, an auricle and a ventricle. (Cka- 



NIOTA.) 



6. Respiration during part or the whole of life performed by means of 

 gills; blood cold. (Ichthyopsida.) 



