and worms. There is a very obvious character by which these may be recognized: they have all bodies composed of numerous segments, and their limbs are jointed, and they have mostly an external skeleton from which their limbs are developed. Sometimes the segments of their bodies are numerous, as in the centipede, lobster, &c.; sometimes several segments are fused together, as in the crab; and sometimes, as in worms, they are indicated by slight markings or depressions of the skin, which give the appearance of little rings, and hence the worms have been named Annelida, or Annulata, or Annulosa. In these last-named cases the segmental nature of the type is detected in the fact that the worms grow, segment by segment; and also in the fact that in most of them each segment has its own nerves, heart, stomach, &c.—each segment is, in fact, a zöoid.[1]
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Fig. 18.
Nervous System of Sea-Hare (A) and Centipede (B).
Just as we recognize a vertebrate by the presence of a backbone and internal skeleton, we recognize an articulate by its jointed body and external skeleton. In both, the nervous system forms the axis of the body. The Mollusc, on the contrary, has no skeleton, internal or external;[2] and its nervous system does not form an axis. As a rule, both vertebrates and articulates have limbs—although there are exceptions in serpents, fishes, and worms. The Molluscs have no limbs. Backboned,—jointed,—and non-jointed,—therefore, are the three leading characteristics of the three types.
