Those animals which possess lateral bundles of bristles (technically termed "chaetae") for use in locomotion constitute the group of "Bristle-worms," or CHAETOPODA. The body of these animals is made up of a preoral lobe or prostomium, and a number of more or less distinct segments following one another in a line, and repeating one another in their internal and external structure. The Chaetopoda embrace the following smaller groups or Orders:—I. Archiannelida, II. Polychaeta, III. Myzostomaria, IV. Oligochaeta. The Archiannelida, although without the characteristic chaetae, are yet anatomically so similar to the true Chaetopoda that they must be included in the group, just as certain fishes are classed as "Vertebrata," although they do not possess vertebrae. The old term Annelida is sometimes used to include the above-mentioned groups, together with the Gephyrea[292] and the Hirudinea or leeches. ORDER I. ARCHIANNELIDA. The Archiannelida are very simple worms, but simplicity may be, and very frequently is, the result of degeneration; and it is not always possible to determine whether a simple animal is primitively, _i.e._ ancestrally simple, or whether it is secondarily simplified. Hence the term _Haplodrili_ has been employed by Professor Lankester as the name of the group; a term which does not prejudge the question as to whether or not the worms are {242}"primitive." It is quite possible, and even probable, that _Dinophilus_ is ancestrally simple; whilst many features in _Polygordius_ appear to be the result of simplification. For this reason it would be well to separate _Dinophilus_ from the other two genera, on account of its much less elaborate and more generalised structure,—so generalised, in fact, that the worm is by some authorities placed amongst the Planarians; for the present, however, the group Archiannelida may be regarded as containing three genera: _Dinophilus_, _Protodrilus_, and _Polygordius_.[293] [Illustration: FIG. 121.—_Dinophilus taeniatus._ (From Harmer.) The left figure represents the dorsal surface of a young individual, × 76; the mouth and alimentary tract are seen by transparency: _p_, prostomium, with two bands of cilia and a pair of eyes; _a_, anus; _t_, tail; 1 to 5, the segments with ciliated bands. The right figure shows the anatomy of the male, × 38: _b_, rectum; _c_, body-cavity; _d_, vas deferens; _m_, muscular organ (pharynx); _n'_, the first nephridium; _oe_, entrance to oesophagus; _p_, penis; _st_, intestine (stomach); _s.x_, seminal vesicle (5th nephridium).] _Dinophilus_ is represented on our coasts by at least two species: _D. gigas_ Weldon[294] and _D. taeniatus_ Harmer.[295] The latter is about one-twelfth of an inch in length, bright orange in colour, and more or less abundant, at springtime, in the rock pools around Plymouth, where it may be found amongst green algae, or on the mud at the bottom of the pools.