CHAPTER IX
THE CHAETOPODOUS WORMS—THE ARCHIANNELIDA—ANATOMY OF NEREIS, AS TYPICAL OF THE POLYCHAETA
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 "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]]
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.
The animal consists of a broad prostomium, with a pair of eyes; and of a body, distinctly constricted in immature specimens into five or six segments, followed by a short conical tail. There are neither chaetae nor tentacles; locomotion is chiefly effected by means of the bands of cilia which encircle the body in a regular fashion, two bands round the head, and two round each segment in D. taeniatus; in some species there is only a single band on each segment. The whole of the ventral surface is covered with cilia, by the aid of which the animal probably "creeps" along the weeds.
The alimentary canal is straight, and divisible into the regions shown in Fig. 121; a muscular protrusible organ, which is a ventral outgrowth of the foregut, is employed as a "sucker." The coelom is more or less obliterated (or ill developed). The excretory system in the genus is varied: in some species, as in D. gigas, it is stated to be constructed on the Planarian plan; in others, as in D. taeniatus, the organs are definite nephridia. Of these tubes there are five pairs, the last pair in the male serving as a seminal vesicle. Each nephridium is a ciliated tube, the internal end of which lies in the body-cavity and appears to be blocked by a ciliated tongue-shaped appendage. The first pair corresponds to the "larval nephridia" of Trochosphere larvae.