GENERAL CHARACTERISTICS. 
3 
from the Vertebrates in the arrangement of some of the principal organs of the 
body. For instance, although as in Cliordates the front end of the nervous chord 
is lodged in the head above the mouth, and constitutes the brain, the rest of it 
runs along the ventral or lower surface of the body beneath and not above the 
alimentary canal, which thus, in its anterior or oesophageal part, passes right 
through a ring or collar of the nervous system. Again, the chief centre of the 
circulation, the heart, is lodged in the back and not in the lower part of the body, so 
that the arrangement of these two structures is exactly the opposite of that which 
obtains in the Chordata. If, for example, a transverse section be cut through a 
fish a little behind the head, the nerve-chord, the alimentary canal, and the heart 
will be found to occupy the following positions—the first named being in the back, 
the second in the middle, and the third below; while, on the contrary, a section of 
the same kind, taken in substantially the same place in a centipede, will show that 
the heart is above, and the nerve-chord below the alimentary canal. 
This arrangement of the organs in question does not, however, exist in all 
invertebrated animals. In some the nervous system is absent; in others it con¬ 
sists of two strands, one running along each side of the body, and neither above 
nor below the alimentary canal. In others, again, there is no circulatory system, 
and in others no alimentary canal. There is consequently an extreme divergence 
in anatomical structure between various kinds of Invertebrates, and zoologists 
have attempted to express these differences, as explained above, by the referring 
these various creatures to distinct subkingdoms. 
Eight of such subkingdoms are provisionally recognised in the present work, 
and are arranged as follows:—(1) Arthropoda,or Invertebrate animals with jointed 
legs, such as insects, spiders, and crustaceans; (2) Echinodermata, or star-fish, sea- 
urchins, stone-lilies, etc.; (3) Mollusca, or soft-bodied, unsegmented animals, often 
with a shell, but without legs, like cuttle-fish, whelks, and oysters; (4) Molluscoida, 
including the lamp-shells and corallines; (5) Vermes, or worms and their 
kindred; (6) Ccelenterata, or jelly-fish, sea-anemones, and corals; (7) Porifera, or 
sponges; and (8) Protozoa, or single-celled animals, like the microscopic foramin- 
ifera. As the special characters of each of these subkingdoms are pointed out in 
the chapters devoted to them, no further reference is necessary in this place. 
special The term Arthropoda is applied to the classes of animals com- 
Characters of posing this subkingdom in allusion to the fact that the limbs are 
Arthropods. c ji v i c ] ec | by joints into a series of movable segments. The title, how¬ 
ever, is not in all respects satisfactory, seeing that members of other groups, 
mammals and birds for instance, also have jointed legs, and in one important 
though not typical class of Arthropoda, namely, the Prototracheata, containing the 
aberrant family Peripatidce, the appendages are short and undivided. The name 
is consequently often superseded by the later but more appropriate term Gnatho- 
poda, meaning foot-jawed, which refers to a characteristic that is perfectly 
distinctive of all the species included under the heading. This is the transforma¬ 
tion into jaws, or gnathites, as they are sometimes called, of one or more pairs of 
the appendages that lie at the sides of the mouth, or just behind it. The number 
of pairs involved in the formation of jaws varies from one to six, the smallest being 
found in Peripatus , and the largest in crabs and their allies, while between these 
