THE INSECTS 2965 



With this briet resume of the fundamental features of Chordate morphology, 

 we may turn to the remaining groups of animals, the so-called Invertebrata, which, 

 as a whole, may be distinguished from Chordates merely by negative characteristics, 

 there being no pharyngeal slits, no notochord, and no central nervous system run- 

 ning along the back. Nevertheless, some of the higher groups of invertebrated 

 animals such as the Arthropods and Mollusks resemble each other, and differ 

 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 Chor- 

 data. 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 mid- 

 dle, 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 consists 

 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 referring these 

 various creatures to distinct subkingdoms. 



Eight of such subkingdoms are provisionally recognized in the present work, 

 and are arranged as follows: (i) Arthropoda, or Invertebrate animals with 

 jointed legs, such as insects, spiders and crustaceans; (2) Kchinodermata, or star- 

 fish, sea urchins, stone lilies, etc.; (3) Mollusca, or soft-bodied, unsegmented ani- 

 mals, often with a shell, but without legs, like cuttlefish, whelks, and oysters; (4) 

 Molluscoidea, including the lamp shells and corralines; (5) Vermes, or worms and 

 their kindred; (6) Coelenterata, or jellyfish, seaanemones, and corals; (7) Porifera, 

 or sponges; and (8) Protozoa, or single-celled animals, like the microscopic foramin- 

 ifera. As the special characteristics of each of these subkingdoms are pointed out 

 in the chapters devoted to them, no further reference is necessary in this place. 



. The term Arthropoda is applied to the classes of animals composing 



acteristics *^ s su bkmgdom in allusion to the fact that the limbs are divided by 

 of Arthro- joints into a series of movable segments. The title, however, is not 

 pods in all respects satisfactory, seeing that members of other groups, mam- 



mals and birds for instance, also have jointed legs, and in one import- 

 ant though not typical class of Arthropoda, namely, the Prototracheata, containing; 

 the aberrant family Peripatidce, the appendages are short and undivided. The 



