Annulata or Chætopoda.—So-called because consisting of a series of rings, and being provided with chætæ or bristles; they are to be associated with the Arthropoda, under the one head “Appendiculata,” a better name than “Articulata,” since Cuvier did not include worms in his group. The creatures that are most familiarly called worms are to be found in Case III.; here are a few examples of the numerous kinds of worms that are found living freely in the sea, of earth and freshwater Worms, and of Leeches. All these worms are distinctly characterized by the fact that they consist of a number of definite rings (somites), whence they have been called Annulata. The marine Worm and the Earthworm differ from the Leech in that these rings are provided with chætæ or bristles, of which there are a number in each bundle in the marine, and a few only in the terrestrial or freshwater form: hence the marine Worms are called Polychæta and the latter Oligochæta.
The former are divisible into two great groups. There are those that are free-swimming and are able to forage for themselves, such as the lovely Sea-mouse (Aphrodite aculeata), the large Eunice gigantea, the common Nereis pelagica, or the exquisitely coloured Chloeia flava. Others live a more retired life, dwelling in tubes, which they fashion for themselves; they lead either a solitary or a social life. Here we have examples of Sabella, Sabellaria, Serpula; a number of forms of worm-tubes, showing their great variety and beauty (see especially the delicate Filograna), are to be seen in the small Table-cases placed against the north wall of the Gallery. Attention should be especially directed to Mr. A. T. Watson’s beautiful preparations of Terebella littoralis. We give a figure (Fig. 19) after a drawing by that gentleman of the home of Panthalis oerstedi, the tube-forming habits of which have been carefully observed by him.
Fig. 19.
Home of Panthalis oerstedi.
Fig. 20.
Section across the body of an earth-worm to show the disposition of the more important organs; the body wall (w) consists of dermis, circular, and longitudinal muscles; the body cavity is divided by membranes (c) into a series of chambers, in each of which opens the mouth of a coiled nephridium (n). The axis of the cavity is occupied by the intestine (i); above and below it is a longer blood-vessel (v), and below it is also the central nerve-cord (nc).
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.
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.