But the recognition of a mollusc will be easier when you have learned to distinguish it from one of the ARTICULATA, forming the third great division,—the third animal Type. Of these, our vases present numerous representatives: prawns, beetles, water-spiders, insect-larvæ, entomostraca, 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.[16] [Illustration: 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;[17] 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. Let us now glance at the fourth division—the RADIATA,—so called because of the disposition of the organs round a centre, which is the mouth. Our fresh-water vases afford us only _one_ representative of this type—the _Hydra_, or fresh-water Polype, whose capture was recorded in the last chapter. Is it not strange that while _all_ the Radiata are aquatic, not a single terrestrial representative having been discovered, only one should be found in fresh water? Think of the richness of the seas, with their hosts of Polypes, Actiniæ, Jelly-fish, Star-fishes, Sea-urchins, Sea-pens (_Pennatulæ_), Lily-stars (_Comatulæ_), and Sea-cucumbers (_Holothuriæ_), and then compare the poverty of rivers, lakes, and ponds, reduced to their single representative, the _Hydra_. The radiate structure may best be exhibited by this diagram of the nervous system of the Star-fish.[18] [Illustration: Fig. 19. NERVOUS SYSTEM OF STAR-FISH.]