The _Oligochæta_ are represented by the common Earthworm, the influence of which in the formation of mould and in the general ploughing of the soil was carefully investigated by Mr. Darwin; and by the little _Tubifex rivulorum_ (Bloodworm), which owes both its red colour and its ability to dwell in mud, which is so poor in oxygen as to be unfit for respiration, to the same chemical compound as that which gives the red colour to our blood and carries the oxygen of respiration all over the body. [Illustration: _Acanthobdella_: _e_, eyes; _ch_, chætæ; _s_, sucker. ] The _Hirudinea_, or Leeches, are often said to be distinguished from the _Chætopoda_ by the absence of bristles, but, as a fact, _Acanthobdella_ (Figs. 21 and 22) has very well marked bristles. They always have a sucker at the hinder end of the body by which they are attached to their prey; they are found in fresh water (_Piscicola_), on sea-fishes (as _Pontobdella_), or in moist places, as the Leech (_Hirudo_). The last-named has three jaws, armed with as many as ninety denticles. _Trochetia subviridis_ (Land-Leech) is a species which is found rarely and sporadically in England. The Myzostomaria form a division of Polychæta all the members of which live parasitically on Crinoids, and otherwise present great differences in their habits. Some move about freely on the Crinoids they infest, others are more sluggish and rarely move, others produce galls or cysts on their host, and yet others are internal parasites, and live in the alimentary canal. It is of interest to note that there are corresponding degrees of difference between the young and old specimens of the different groups of species. The general organisation of Myzostomaria is shown in the accompanying figure (Fig. 23) in which the dorsal wall of the body is supposed to be transparent so as to allow of the chief internal organs being seen. [Illustration: Fig. 23. Diagram of Myzostomum to show the general form of the body and the marginal extensile cirri (_c_); within these and on the ventral surface are four pairs of suckers, and more internally five pairs of appendages each bearing two hooks; the proboscis (_p_), the digestive tract and its ramifications, and the reproductive organs are outlined as if seen through a transparent wall; _a_, anus. ]