CHAPTER XIII 
THE CHORDATES 
143. General characters,—Up to the present time we have 
been studying the representatives of a vast assemblage of 
animals whose skeletons, if they have any at all, are located 
on the outside of the body. In the corals, the mighty com- 
pany of arthropods, and the echinoderms, it is external. On 
the other hand, we shall find that the animals we are now 
about to consider, the fishes, frogs, lizards, birds, and mam- 
mals, are in possession of an internal skeleton. In some of 
the simpler fishes and in a number of more lowly forms (Fig. 
96) it is exceedingly simple, and consists merely of a gristle- 
like rod, the notochord (Fig. 98, ne), extending the length 
of the body and serving to support the nervous system, which 
is always dorsal. This is also the type of skeleton found in 
the young of the remaining higher animals, but as they grow 
older the notochord gives way to a more highly developed 
cartilaginous or bony, jointed skeleton, the vertebral column. 
In the young of all these back-boned or chordate ani- 
mals, the sides of the throat are invariably perforated to 
form a number of gill-slits. In the lower forms these per- 
sist and serve as respiratory organs, but in the higher ani- 
mals they disappear in the adult. The chordates are thus 
seen to be distinguished by the possession of a dorsal nerv- 
ous cord supported by an internal skeleton and by the 
presence of gill-slits, characters which separate them widely 
from all invertebrates. 
The chordates may be divided into ten classes, seven of 
11 151 
