This order comprises free Annelides,* which possess setigerous foot-tubercles. The respiratory organs are generally in the form of tufts of external branchiae, arranged along the back or the sides of the body. The sexes are distinct, and the young pass through a metamorphosis. This order includes most of the animals which are commonly known as Sand-worms and Sea-worms, together with the familiar Sea-mice.
The integument is soft, and the body is distinctly divided into a number of rings or segments, each of which, in the typical forms, possesses the following structure. The segment consists of two arches, a lower or "ventral arc," and an tipper or "dorsal arc," with a "foot-tubercle" on each side. Each foot-tubercle consists of an upper process, or "noto-podium," and a lower process, or "neuropodium," each of which carries a tuft of bristles, or "setae," (rarely, a single bristle) and a species of tentacle termed the "cirrhus" (fig. 126).
The outer, cuticular layer of the body is generally more or less chitinous, and is often iridescent. Below this is a muscular layer, by which the movements of the animal are effected, and which encloses the "perivisceral cavity." This cavity runs the whole length of the body, and is lined by a special, often ciliated membrane, which is reflected upon the alimentary canal and other internal organs. It is usually more or less subdivided by imperfect partitions, and is filled with an albuminous fluid containing floating corpuscles, and corresponding with the blood. This so-called "chylaqueous fluid," "performs one of the functions of an internal skeleton, acting as the fulcrum or base of resistance to the cutaneous muscles, the power of voluntary motion being lost when the fluid is let out" (Owen).
The anterior extremity of the body is usually so modified as to be distinctly recognisable as the head, and is provided with eyes, and with two or more feelers, which are not jointed, and are therefore not comparable with the antennae of Crustacea and Insects. The mouth is placed on the inferior surface of the head, and is often furnished with one or more pairs of horny jaws, working laterally. The pharynx is muscular, and forms a sort of proboscis, being provided with special muscles, by means of which it can be everted and again retracted. In most there is no distinction between stomach and intestine, and the epithelium of the alimentary canal, like that of the preceding orders, is ciliated.
* Fritz Muller describes an errant Annelide belonging to the Amphino-midae as living parasitically within the shell of the. common Barnacle (Lepas), showing that the members of this group may sometimes lose their free habit.
The pseudohaemal system is well developed, and consists essentially of a long dorsal vessel, and a similar ventral one, connected by transverse branches, and sometimes furnished at the bases of the branchiae with pulsating dilatations. The contained fluid is mostly red, but is yellow in Aphrodite and Polynoe, and in no case contains corpuscles.
Respiration is carried on by means of a series of external branchiae or gills, arranged in tufts upon the sides of the body on its dorsal aspect, along the middle of the body only, or along its entire length; but in some forms the gills may be rudimentary or wanting. From the position of the branchiae, the members of this order are often spoken of as the "Dorsi-branchiate" (or more properly "Notobranchiate") Annelides.
Fig. 132. - Cirrhatulus grandis, an "Errant Annelide," in its living condition.
The "segmental organs," with few exceptions, communicate with the perivisceral cavity internally, and in certain segments they are always specialised to act as efferent ducts for the reproductive organs.
The nervous system in the Errantia has its typical form, consisting of a double gangliated ventral cord, two ganglia of which are appropriated to each segment. The prae-oesopha-geal, or cerebral, ganglia are of large size, and send filaments to the ocelli and feelers.
The sexes in the Errantia are in different individuals, and reproduction is usually sexual, though in some cases gemmation is known to occur. The process of gemmation is carried on by a single segment, and so long as it continues, the budding individual remains sexually immature, though the young thus produced develop generative organs. Thus, there is in these cases a kind of alternation of generations, or rather an alternation of generation and gemmation; the oviparous individuals producing eggs from which the gemmiparous individuals are born; these, in their turn, but by a non-sexual process, producing the oviparous individuals. While the form of gemmation just alluded to has long been known as not uncommonly taking place among the Errant Annelides, no example of continuous gemmation has until lately been recognised in any Annelide. Recently, however, Dr M'Intosh has described a remarkable species of Syllis (S. ramosa), which inhabits a Hexactinellid Sponge from the Philippines, and in which the thread-like body is intricately branched, giving off lateral offsets, and thus becoming a truly composite organism. This singular form is further remarkable in the fact that no traces of a head have hitherto been discovered, so that it is probable that the entire branched organism possessed but a single head.
Not only does gemmation occur among the Errant Annelides, but, in a few instances, fission has been noticed to take place. Occasionally, also, the males and females differ from one another, and both may differ from the sexless forms, when these exist. Thus, Heteronereis is founded upon the sexless forms of Nereis; whilst the species of the genus Autolytus, amongst the Syllidea, exhibit a still more remarkable polymorphism, the males and females being extremely dissimilar, and there being in addition a third sexless form, which produces the sexual individuals by gemmation at its hinder extremity.
The embryo usually appears, on its liberation from the ovum, as a free-swimming, ciliated body, possessing a mouth, intestine, and anus. The cilia are primarily diffused, but become aggregated so as to form a single median belt, or two bands, one about each extremity, or a series of bands (fig. 131, B and C). The head, with its feelers and eye-specks, appears at one extremity, whilst the segments of the body begin to be formed at the other. Each segment is developed in four parts, the two principal ones forming half-rings, united by shorter side-pieces, from which the setigerous foot-tubercles are developed. The ciliated band or bands finally disappear, and new rings are rapidly added by intercalation between the head and the segments already formed.
Amongst the best known of the Errantia is the common Lob-worm (Arenicola piscatorum, fig. 133, C), which is used by fishermen for bait. The Lob-worm lives in deep canals, which it hollows out in the sand of the sea-shore, literally eating its way as it proceeds, and passing the sand through the alimentary canal, so as to extract from it any nutriment which it may contain. It possesses a large head, without eyes or jaws, and with a short proboscis. There are thirteen pairs of branchiae placed on each side in the middle of the body.
Fig. 133. - Errant Annelides. A, Hairy-bait (Nephthys); B, Sea-mouse (Aphrodite); C, Lob-worm (Arenicola). (After Gosse.)
In the Sea-mouse (Aphrodite, fig. 133, B), the back is covered with a double row of membranous imbricated plates, which are called "elytra," or "squamae," and respiration is effected by the periodical elevation and depression of these plates, whereby water is alternately admitted into, and expelled from, a space beneath them. This space is separated by a membrane from the perivisceral cavity below, and contains the gills in the form of small fleshy crests. The pharynx is thick and muscular, and can be everted like a proboscis, and the intestine has a number of lateral branched In the Nereidae, or "Sea-centipedes," the body is greatly elongated, and consists of a great number of similar segments, with rudimentary branchiae.
The head is distinct, and carries eyes and feelers, whilst the mouth is furnished with a large proboscis, and often with two horny jaws. In the Eunicea the branchiae are usually well developed and of large size, and the mouth is armed with seven, eight, or nine horny jaws. Eunice gigantea attains sometimes a length of over four feet, and may consist of more than four hundred rings.
All the Errant Annelides are marine, occurring in all seas from the Arctic Ocean to the equator, and extending to great depths. A few forms (e. g, Tomopteris) are pelagic. Others live in sand and mud; whilst others hide under stones, or in fissures in rock-pools; and others, again, bore holes in calcareous rocks. A few live as "commensals" on other animals.