Sub-Order 3. Terebelliformia.—The prostomium is a more or less prominent lobe (upper lip) with or without tentacles but without palps. The peristomium may carry cirri or "tentacular filaments."[[365]] The parapodia are feebly developed; there are no ventral cirri; the dorsal cirri may exist and function as gills on more or fewer of the anterior segments. The chaetae are unjointed, and uncini are usually present. The buccal region is not eversible; there are no jaws. The septa are usually incomplete, with the exception of one strongly-developed "diaphragm" anteriorly; the nephridia are dimorphic, those of the anterior (prediaphragmatic) segments are of large size and are excretory; the posterior series are mere funnels, and act as genital ducts. These worms are burrowers or tube-formers, and in the majority the tube-forming glands are grouped on the ventral surface of the anterior segments to form "gland-shields."
Sub-Order 4.—The Capitelliformia have no prostomial processes, but possess a pair of large retractile "ciliated organs." The parapodia do not project; the chaetae are unjointed, and are hair-like in the anterior segments and hooded "crotchets" posteriorly; this external division of the body does not correspond with definite internal differences. There are no cirri, though special "gills," often retractile, are frequently present. The buccal region is eversible; there is no armed pharynx. An "accessory gut" or "siphon" exists. The nephridia are small, and sometimes more than one pair in a segment; special genital funnels exist in more or fewer of the anterior segments of the hind body. There is no system of blood-vessels; the coelomic corpuscles are red. The worms are burrowers.
Sub-Order 5.—The Scoleciformia possess a prostomium, which rarely (Chlorhaemidae) carries any sensory processes; the peristomium is without cirri (except, perhaps, in the Chlorhaemidae). The parapodia are ill developed, and may be absent; only rarely are dorsal cirri present, acting as gills; ventral cirri are absent. The chaetae are unjointed; true uncini are not present. The buccal region is eversible, but there is no armed pharynx. The septa are not regularly developed, as more or fewer are absent, and the nephridia are considerably reduced in number, it may be to a single pair (Sternaspidae and some Chlorhaemidae), but they are all alike.[[366]] The worms are mostly burrowers.
The branch Cryptocephala contains two sub-Orders:—
Sub-Order 1. Sabelliformia.—The prostomium is entirely hidden by the forward extension of the peristomium; the tentacles are very small, being frequently represented merely by small knobs of sense-cells; the palps, on the other hand, are greatly developed, branched, and contain blood-vessels, acting as respiratory as well as sensory organs. The peristomium never carries cirri or chaetae, and it is usually raised up into a projecting collar, used in fashioning the lip of the animal's tube. The parapodia are but feebly developed; cirri are absent, except in the Serpulidae, where the dorsal and ventral cirri become united to form the "thoracic membrane" (Meyer). The chaetae are of two kinds—unjointed, hair-like, fringed bristles and "uncini." By their arrangement the body is divided into a thorax of nine segments and an abdomen; in the former the capillary chaetae are dorsal, and in the latter ventral. The buccal region is not eversible; there is no pharynx. The septa are regularly developed in the abdomen, but are absent in the thorax; the nephridia are dimorphic; there are two large ones in the thorax opening by a median dorsal pore just above the brain; those of the abdomen are small funnels, and act as genital ducts. The worms are tubicolous; "gland-shields" are present on the thoracic segments.
Sub-Order 2. Hermelliformia.—The peristomium (Fig. 135) is enormously developed, and forms a bilobed hood capable of closing over the mouth; the truncated free end of each lobe carries three semicircles of peculiar chaetae, which act as an efficient protection when the worm is withdrawn into its tube. The prostomium is very small, but retains a pair of well-developed tentacles; the palps, which are subdivided as in the Sabelliformia, have become fused with the ventral edges of the peristomium, and appear as a series of ridges on each side, carrying numerous filaments. The thorax consists of five segments, the notopodia of three of which are well developed and bear strong chaetae; dorsal cirri are present along the greater part of the body, and act as gills. The arrangement of the chaetae and of the internal organs is as in the Sabelliformia. The worms form tubes of sand.
BRANCH A. PHANEROCEPHALA.
Sub-Order 1. Nereidiformia.[[367]]
Fam. 1. Syllidae.—These are small worms, the majority being less than an inch long, so that they are not easily observed. The body consists of a fair number of segments.[[368]] In many genera a dorsal bundle of unjointed, natatory chaetae makes its appearance at maturity. The palps, which are grooved, are in some cases so united with one another and with the prostomium as to be scarcely recognisable. (For head see p. [262], and for feet see p. [264].) The pharynx is armed with one or more teeth. There is a special gizzard, following the pharynx, and provided with thick, muscular walls of peculiar structure. Following the gizzard, the oesophagus receives in many genera a pair of T-shaped diverticula, that are used for storing water, which is swallowed with food. These diverticula are absent in Autolytus and other free-swimming forms. The reproduction of the members of this family is interesting, and has already been described (p. [278]).
