THE VERTEBRATE ANIMALS 



OF THE 



1: 



NORTHERN UNITED STATES. 



VERTEBRATA. (The Vertebrates.) 



The Vertebrates are, in popular language, " animals with a 

 back-bone." They are distinguished from all other animals, says 

 Professor Huxley, " by the circumstance that a transverse and 

 vertical section of the body exhibits two cavities, completely sep- 

 arated from one another by a partition. The dorsal cavity contains 

 the cerebro-spinal nervous system ; the ventral, the alimentary 

 canal, the heart, and, usually, a double chain of ganglia, wbieb 

 passes under the name of the ' sympathetic' A vertebrated ani- 

 mal may be devoid of articulated limbs, and it never possesses 

 more than two pairs. These are always provided with an internal 

 skeleton, to which the muscles moving the limbs are attached." 



Modern researches have shown that, besides the ordinary " back- 

 boned animals," certain other creatures, formerly considered as 

 Mollusks or Worms, are really degenerate forms of Vertebrates, 

 and must be considered as members, or at least as associates, of 

 this group. The resemblance to the other Vertebrates on the part 

 of the forms in question is seen in their early or larval develop- 

 ment, and scarcely at all in the adult condition. " Many of the 

 species start in life with the promise of reaching a point high in 

 the scale, but after a while they turn around, and, as one might say, 

 pursue a downward course, which results in an adult which dis- 

 plays but few resemblances to the other vertebrates." (Kingsley.) 

 These are the Tunicates or Ascidians, forming the Class or Prov- 

 ince of " Urochordata." The essential character of the Vertebrata, 

 in the broad sense of the term, is now understood to be this : " The 



