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. [Illustration: Fig. 19. Home of _Panthalis oerstedi_. ] [Illustration: 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_). ]