GENERAL CHARACTERISTICS. 3 



from the Vertebrates in the arrangement of some of the principal organs of the 

 body. For instance, although as in Chordates 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 ^^ e 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, divided 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 



