The septa are well-developed in Oligochaeta (except Aeolosoma), Errantia, and many Tubicola, whilst in other Tubicola only one or two may be present in the anterior region of the body. The chambers thus made communicate either round the intestine, round the ventral vessel, or by pores variously placed. They are provided with dorsal pores opening outwards in many Oligochaeta2. The muscular tissue is composed of cells with a superficial fibrillar striated layer, and usually but a slight remnant of protoplasm surrounding the nucleus. A nucleated connective tissue intervenes between the muscle-cells. It is largely developed in the anterior region of the body in many Serpulidae and reduces the coelome to two narrow passages. The coelomic surface of the body-walls as well as of all the internal organs appears to be covered by an epithelium, which varies in character not only in different regions of the body but in different Chaetopods3. This coelomic epithelium is ciliated in the Aphroditidae, polycirrine Terebellidae, and the Anangian Glyceridae.
1 In Filograna and Apomatus the peduncle of the operculum bears lateral processes, and the stopper is represented by a terminal sphere. For the metamorphosis of the filament, cf. F. Muller, Facts for Darwin, 1869, p. 112.
2 Claparede thought that the material for the tube was secreted entirely by the most anterior pair of nephridia, the only pair, as he supposed, present in Serpulidae. Pruvot observed the posterior part of a Myxicola, accidentally divided, envelope itself in a tube of mucus, undoubtedly derived in this instance from the hypodermic glands. The presence of the characteristic rods alike in the gland cells and in the substance of the tube of Sabella points to the same conclusion. It is also the natural conclusion judging from the analogy of Nemertea. Aggregations of unicellular glands open on the ventral surface of certain segments in Polydora, and, according to Jacobi, secrete material for the tube.
The nervous system consists of a pair of cerebral or supra-oesophageal ganglia and a ventral nerve cord, the oesophageal commissures connecting them and a variously developed stomato-gastric system in connection with the pharynx. The dorsal surface of the cerebral ganglia, the ventral surfaces of the ventral ganglia are in contact with the hypodermis in many Polychaeta. The posterior termination of the nerve-cord in Telepsavus costaricm, the whole nervous system of a Terebella sp. ?, from Heligoland (Semper), and of Aeolosoma (which is reduced to the cerebral ganglia), are included in it as in certain remarkable and primitive worms 4. The cerebral ganglia are simple in Oligochaeta, complex in many Polychaeta in which they consist of antennary and stomato-gastric centres, the former supplying the antennae, the latter the palpi or their branchial homologues in some Tubicola (cf. note, p. 593). The first two or three ventral ganglia may be united5, the remainder are usually distinct. The ventral cord of some Tubicola (Polydora, Serpulidae) is ladder-like, the right and left half of each ganglion being widely separate and united by transverse commissures.
The longitudinal commissures between successive pairs of ganglia are separated in Phyllodocidae, but as a rule they are contained within a common sheath. The nerves given off laterally from the ganglia to the parapodia sometimes possess a small ganglion where they branch. The ganglion cells may be spread over the whole extent of the cords, as in Liimbriculidae and Lumbricidae among Oligochaeta, and in the anterior part of the cords in some Polychaeta, e. g. Nephthys, but they are usually aggregated in the ganglia. The stomato-gastric system is present in all Oligochaeta (p. 211) and most Polychaeta. In the latter it originates from the cerebral ganglia (e. g. Eunicidae, Serpulidae), from the same ganglia and the oesophageal commissures as well (e. g. Nephthys, Phyllodoce), or from the commissures alone (e. g. Ophelia). This last mode of origin obtains in all Oligochaeta. It is best developed in those Polychaeta which possess a muscular pharynx1. Supporting or skeletal structures, the so-called 'giant-fibres' or 'neurochord,' are found in nearly all Oligochaeta and many Polychaeta on the dorsal aspect of the ventral cords (p. 211). For the lateral ganglionic cords of Oligochaeta, see p. just cited.
1 Bands of muscles are stated to pass across the coelome beneath the intestine, and from one parapodium to another in Glyceridae.
2 See on these pores p. 199, ante. Add to what is there stated that the position of the first pore appears to constitute a specific character, and that in one instance at least the worm possesses, when irritated, the power of expelling through them a jet of coelomic fluid to the height of two feet. See Vorderman, Tijdschr. fur Nederl. Indie (8), ii. 1882 (Naples Zool. Jahresbericht, 1882, 'Vermes.' P- 273)- Our English Earthworm suffers coelomic fluid to escape sometimes if pressed by the fingers.
3 See on Arenicola and Lumbricus, Viallanes, A. Sc. N. (6), xx. 1886.
4 I.e. the Archi-Chaetopoda, p. 609, and Archi-Annelida, p. 613.
5 In Ophelia (Tubicola) a ganglion is situated on each oesophageal commissure, and supplies the first pair of feet (Pruvot).
Organs of special sense are found in the form of ciliated grooves, tactile cells or bodies, eyes, and otocysts. Ciliated grooves occur on the prostomium of the Oligochaete Aeolosoma, and many Polychaeta, and are supplied by nerves from the cerebral ganglia. Their function is unknown; it is possibly olfactory2. Tactile hypodermic cells furnished with external setae, long or short, and connected basally with a nerve filament, derived either from a nerve or a ganglion cell are probably widely distributed. For their occurrence in Oligochaeta, see p. 211. In Polychaeta they occur aggregated on papillae of the antennae and the cirri, and of the elytra of Aphroditidae, and as special organs in the 'goblet' organs of the Capitellidae where they are scattered over the proboscis, prostomium, and thorax; or in Polyophthalmus as a single pair in connection with the ciliated grooves; and as 'lateral' organs, one between each noto- and neuro-podium in the Capitellidae, or the corresponding bundles of setae of Polyophthalmus. The nerves to the goblet and lateral organs end in ganglia. Eyes are absent in all Oligochaeta except some Naidomorpha. They consist in Nats proboscidea of a few large cells covered on one side by small pigmented cells.