In a few cases, such as the Chlorhaemids and Sternaspis, and to a slight degree in Arenicola, the "head" and even the anterior part of the worm is capable of being withdrawn into the body.
The Parapodia and Chaetae.—The typical parts of a parapodium have been described in the preceding chapter; here it is only necessary to refer to the series of diagrams (Figs. 136, 137) representing the parapodia of the more common Polychaetes, and to add a few remarks about them.
Fig. 136.—Parapodia. A, Nephthys; B, Amphinome; C, Glycera (the unlettered lobe above g is the notopodial cirrus); D, Syllis; E, Eunice; F, Phyllodoce. a, Notopodial cirrus; b, notopodium; c, neuropodium; d, neuropodial cirrus; g, special gill; n, aciculum (omitted in B); x, cirriform lip of chaetigerous sac.
In most Annelids the chaetae are in two bundles on each side, but there are certain families in which the dorsal bundle, and even the notopodium itself, is absent, as in the Eunicidae, Syllidae, and Phyllodocidae; or the dorsal bundle may be absent only in certain regions of the body, as in the hind-body of Terebellids. In some Amphinomidae and Aphroditidae the notopodium is scarcely distinct as a separate lobe, being a slight tubercle on the upper surface of the neuropodium; but the notopodial chaetae are present, and indeed particularly well developed in many cases.
But whilst, in the Nereidiformia, the parapodia, whether consisting of two lobes or only one, are always well developed, and project to a more or less pronounced degree from the sides of the body, it is otherwise in the rest of the group, where the chaetigerous lobes are usually reduced to mere tubercles or ridges, no doubt in relation to their burrowing or tubicolous habits. In Sternaspis the chaetae issue directly from the body-wall.
Amongst the Nereidiformia we find examples in which the parapodia, instead of being more or less conical "legs," are flattened fore and aft so as to serve as efficient "fins," as in the active swimmers, Nereis virens and Nephthys caeca, and in the pelagic Phyllodocids, Alciopids, Typhloscolecids, and Tomopteris.
Fig. 137.—Parapodia. A, Polynoë; B, Scoloplos; C, Euphrosyne. (Transverse section of body.) a', Accessory cirrus; y, doubtful branchiae; D, Sabella (thoracic). a, Notopodial cirrus ("elytron" in A, "gill" in B); b, notopodium; c, neuropodium; d, neuropodial cirrus; n, aciculum (accidentally omitted in C).
Of the typical dorsal and ventral cirri, the ventral is only absent in some Amphinomids amongst the Nereidiformia; the dorsal is absent in Nephthys and degenerate in Glycera, whilst in a very large number of families of the other sub-Orders neither cirrus is present. These cirri, though originally filamentous and sensory, may, by virtue of special blood supply, become "gills," and this occurs in several families of different sub-Orders. Thus in Eunice this gill is comb-like; in Amphinome and in Arenicola (on certain segments) it is arborescent, as it is also in one to three segments in Terebellids; whilst in Ariciidae, Spioniformia, Cirratulidae, Opheliidae, and Sabellaria it remains more or less finger-shaped or filamentous. In the family Serpulidae the thoracic cirri, both dorsal and ventral, become flattened and extended antero-posteriorly, and unite with one another to form the "thoracic membrane."[[320]] In Phyllodocidae the cirri are foliaceous and natatory, and they contain a great quantity of glands of a peculiar character. The Aphroditidae are distinguished from other Annelids by the possession of "elytra" or dorsal scales, which appear to be the dorso-ventrally flattened cirri, retaining their sensory nature, but adding to this function several others.[[321]]