. Natural history. Zoology. 682 VERMES—CLASS ROTIFERA. alimentary canal of the animal from which they set out. The migrations of other species is substantially similar to that of E. gigas. For instance, E. polymorphns lives as a mature animal in the intestine of ducks or other water-fowl, while in the immature stage it occurs in the little fresh-water crustacean (Gammarus pulex). This amphipod (Cr. pulex) is also, in conjunc- tion with the fresh-water wood-louse (Asellus aqvaticits), the intermediate host of two other species, JE. angustatus and JS. proteus, which, when mature, infest the alim
. Natural history. Zoology. 682 VERMES—CLASS ROTIFERA. alimentary canal of the animal from which they set out. The migrations of other species is substantially similar to that of E. gigas. For instance, E. polymorphns lives as a mature animal in the intestine of ducks or other water-fowl, while in the immature stage it occurs in the little fresh-water crustacean (Gammarus pulex). This amphipod (Cr. pulex) is also, in conjunc- tion with the fresh-water wood-louse (Asellus aqvaticits), the intermediate host of two other species, JE. angustatus and JS. proteus, which, when mature, infest the alimentary canal of fresh-water fish. Instances of the occurrence of worms of this group in mankind have been recorded, but, happily, such cases are rare. Although, probably, more nearly related to the Thread-worms than to any other group in the animal kingdom, the Arrow-worm (Sagitta) presents so many peculiarities of organisation that its true systematic positioii is still a matter of doubt, and it forms the sole type of an order known as Chaetognatha. This name, meaning "bristle-jawed," is given to it in allusion to the fact that the head is armed with a number of strong, movable, horny spikes which act as jaws. The body is long, slender, and unsegraented, with a rounded, blunt head, bearing eyes, and a narrow-pointed tail ; but the tip of the tail and the sides of the body in its hinder half are furnished with horizontally- projecting fins (Fig. 9, b). The Arrow-worms, so called from their arrow-like shape, are not parasitic like the Thread-worms, but are to be found swimming in numbers on the surface of the ocean in all parts of the world. THE WHEEL-ANIMALCULES. Class Rotifbka. The Rotifera, or " Wheel-animalcules," as they were formerly called, on account of the appearance in the water of a rapidly rotating wheel, to which the movement of the circlet of hairs on their heads gives rise, are all small, aquatic animals, varying in length from J to j-J^y of
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