THE VERTEBRATE ANIMALS 
OF THE 
NORTHERN UNITED STATES. 
VERTEBRATA. (THE VERTEBRATES.) 
Tue 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, which 
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 
