Fig. 139.—Aphrodite. Foot, × 2. a, Elytron; b, notopodium; c, neuropodium; d, neuropodial cirrus; n, aciculum; 1, iridescent bristles; 2, stiff chaetae; 3, felt.
Gills.—We have already seen that several different organs, e.g. the palps in Sabelliformia, the prostomial tentacles of Chlorhaemidae, and the notopodial cirri of sundry other Polychaetes, may take on a respiratory function. There are, however, certain "gills" developed either on the parapodium itself or elsewhere on the body which it is difficult to homologise. Such are the retractile gills on the parapodia of the Glyceridae (Fig. 136, C); those of Dasybranchus, near the abdominal neuropodia; those of Mastobranchus, near the notopodia. Nephthys has a sickle-shaped gill on the under surface of the notopodium. The long gill filaments at the posterior end of Sternaspis, again, are only doubtfully interpreted as the dorsal cirri of some of the posterior segments.
Since primitively the whole skin of the worm is respiratory, any part of the skin may become more or less specialised for this function, and chiefly, of course, on the more actively moving parapodia. The blood-vessels constituting the essential part of the "gill" may make use of any already existing outgrowth (such as a cirrus or a tentacle), or may push the body-wall out on their own account.
Internal Anatomy.
Probably those organs which have the greatest effect in modifying the shape of the body are the septa, for we find in the long, free-swimming worms that these are regularly present throughout the body, and external "segmentation" of the body is well marked. In burrowing and tubicolous forms the septa are frequently incompletely developed, or more or fewer may be absent; and the body becomes less distinctly segmented externally, tends to vary greatly in diameter during movement, or becomes plumper. With the disappearance of the septa there is also a diminution in the number of nephridia, as in Arenicola, with only six pairs. Further, there is frequently a dimorphism of these organs; instead of all of them serving equally as excretory organs and as genital ducts, some of the most anterior in the Sabelliformia and Terebelliformia become greatly enlarged, and take on practically the whole of the former function; whilst more or fewer of the posterior nephridia dwindle in size, and become genital ducts. The absence of septa allows a free communication between the successive segments, and thus a freer flow of coelomic fluid for the distension of the anterior end of the worm during burrowing.
The alimentary system presents certain modifications of a systematic value. In the Nereidiformia the muscular pharynx, which is always protrusible and is preceded by an eversible buccal region, frequently encloses thickened cuticular plates which serve as crushing and grasping organs. The form, number, and arrangement of these "jaws" vary in the different families. They form valuable fossil records of extinct worms.
In the Scoleciformia and Capitelliformia the buccal region exists, but there are no jaws. In the Sabelliformia and Terebelliformia eversion does not take place and jaws are absent.
Amongst the Nereidiformia the jaws are absent in the Phyllodocidae and Hesionidae; when present they are usually set in the direct course of the food. There may be one small tooth used for stabbing, as in some Syllids (Fig. 141, A); or a circle of such denticles (Autolytus, Fig. 140, D). To these are added powerful grasping jaws in Nereis (E); or the latter may alone be present, as in Glycera (F). In Polynoë the four jaws are carried by hard pieces, to which the muscles are attached (C and G). In Nephthys there is a dorsal and a ventral jaw.
