Multisegmental Vermes, with a more or less prominent prostomial lobe; with locomotor organs in the shape of chitinoid setae, implanted either in the body-zvall or in special elevations, the parapodia; with a body divided into somites by external furrows and typically by internal fibro-muscular septa. Cilia are restricted to special regions or certain organs. The nervous system consists of a pair of cerebral or prostomial ganglia, and a paired ventral cord zvith distinct or indistinct ganglia. The nephridia are typically repeated in each body-somite; the genital organs may be similarly repeated or be restricted to certain somites, usually the anterior. Development is direct or with a me ta morphosis.
Of the two orders, Polychaeta and Oligochaeta, into which the class is divisible, the former is usually distinguished by the presence of para-podia and by appendages to the head and somites, viz. antennae, cirri, and branchiae.
The division of the body into somites is not well-marked in some Polychaeta Tubicola. The number of somites present varies much, and may attain to several hundreds, as in some Polychaeta Errantia and terrestrial Oligochaeta. Annulation. of the somites, as in Hirudinea, is very rare. The head is generally distinct, and consists of a prostomium and \peristomium. The former varies in size, and in some Tubicola and terrestrial Oligochaeta becomes obsolete1. It usually bears in Polychaeta a certain number of appendages, the antennae or tentacles. Two which originate from its inferior aspect, and differ structurally as well as by their innervation from the other appendages, are often termed palpi. The numerous extensile tentacles of the Terebellidae, which contain a prolongation of the coelome, and the gill-processes so-called of the Serptdidae, appear to belong to the prostomium, as they originate in front of the ring of cilia which limits posteriorly the prae-oral lobe of the larva. The peristomium or buccal somite is sometimes very similar to the following somites, sometimes markedly different, and it may coalesce with 1-3 of them2. The body
1The prostomium of the adult Chaetopod is certainly a rudiment of the larval prae-oral lobe, when it contains the supra-oesophageal ganglion. It is, however, in some cases, e. g. some Oligochaeta, an outgrowth of the following somite, if Vejdovsky's statements are right. Cp. p. 197.
2The number of prostomial antennae varies from two to five. When there are five there is a median azygos antenna, a pair of antero- or supero-lateral antennae, and a pair of postero- or infero-lateral; all alike innervated from the cerebral ganglia. The latter are divisible, according to Pruvot (A. Z. Expt. (2), iii. 1885), into an antennary and a stomato-gastric portion. The former supplies the antennae, and where there are five of these structures, as in Eunice, it is subdivisible into a part supplying the median and antero-lateral antennae, and a second part supplying the postero-lateral pair. The stomato-gastric portion innervates the palpi, and gives origin to the stomato-gastric nerves as well. The homology of the Serpulidan branchiae with palpi is proved by their innervation from the stomato gastric portion of the cerebral ganglia. Pruvot's theory that the subdivisions of the either consists of a uniform series of somites, as in Oligochaeta and Poly-chaeta Errantia, or it may be divided into an anterior region, the thorax, and a posterior, the abdomen, with differing somites as in Polychaeta Tubicola. The number of divisions may be greater, as in Chaetopteridae. The terminal, anal, or pygidial somite is often much reduced.
A post-anal cirriform part of the body appears to be present in Nephthys (Pruvot), and in the Oligochaete Criodrilns there are seven abbreviated post-anal somites (Vejdovsky).
The locomotor setae are implanted in Polychaeta in parapodia (infra). They are grouped, as a rule in large numbers, into bundles which are either single or double according as the parapodium is uni- or bi-ramose. The number of setae in a bundle, their size and shape is often characteristic. A stout seta or aciculum very commonly forms the centre of the bundle. Aphrodite among Polychaeta is remarkable for having large numbers of hair-like setae growing from the notopodium. Some of these are iridescent, others form a felt-work over the dorsum. A similar but more sparingly developed felt-work is occasionally present in the allied genus Hermione. The setae of Oligochaeta are grouped in aquatic and some terrestrial forms in a dorsal and a ventral set. In other terrestrial forms there may be four setae, implanted singly on each side of a somite or a complete or incomplete ring of single setae girthing it. The number of setae in a bundle is always small, and in some terrestrial Oligochaeta (Lumbricus, Antetis, etc.) is reduced to two. The dorsal setae of the aquatic Oligochaeta are often remarkably long.
The setae are chitinoid, their shape and size vary remarkably and are adapted frequently to the necessities of the worm, e. g. the hooked ventral setae of many Tubicola, by means of which they creep along their tube. They are moved by special muscles, protrusor as well as retractor, and originate from cellular sacs or trichophores, invaginations of the hypodermis. Each seta in a bundle is the product of a single cell, and either springs from a separate sac or together with others from a common sac. They are rarely entirely absent, as e. g. from the parapodia of Tomopteris. In the Oligochaete Anachaeta the sacs persist, but do not develope setae.
The parapodia of Polychaeta are hollow lateral elevations of the body-wall, either simple (uniramose)1 or divided (biramose) into a dorsal notowithdraws into it. The operculum retains both a vascular and nervous supply and is probably in part respiratory1.
Cerebral ganglia indicate the presence of somites is perhaps scarcely tenable; at least, it is quite as likely that its division into lobes is the consequence of an increase in the number of cephalic appendages.