The Chaetopoda appear to have considerable powers of reparation after injury, and the formation of a new head with anterior somites, and of new posterior somites has been observed. Phosphorescence occurs in some terrestrial Oligochaeta and some Polychaeta. The light is sapphirine, violet or green; it appears either over the surface of the body in general, or in certain limited spots, e.g. on the elytra of Polynoe, and is due in some cases to the secretion of unicellular hypodermic glandsl.
The Oligochaeta are either freshwater or terrestrial. The Polychaeta are marine2, but a freshwater Lumbriconereis and a Nereid have been found in Trinidad (von Kennel), and a freshwater tubicolous genus, Mana-yunkia, occurs in N. America (Leidy). The Chaetopoda are free except some Tubicola which fix their tubes to stones, shells, or to living animals, e. g. Serpula and Spirorbis to Crabs; Branchiomma vigilans to Aphrodite; and various others to Starfish (Astropecten), or on Sea Urchins (Cidaris). A few Polychaeta are pelagic (Alciopidae, some Phyllodocidae; Tomopteris; Typhloscolex): others only at the reproductive season (Heteronereis, Syllidae). Certain Tubicola bore into rocks or shells (Heterocirrus = Dodecaceria, Polydora ciliata, Sabella saxicava). Most Errantia are carnivorous; the Tubicola and Oligochaeta are vegetable feeders. The only instances of parasitism are the Eunicid Oligognathus Bonelliae in the coelome of the Gephyrean Bonellia, Alciopina parasitica (? a larval Alciope) in a Pleurobrachia (Ctenophora), an Amphinomidan in the branchial cavity of Lepas anatifera (Cirripidid)3, and the family Myzostomidae (infra, p. 609). Commensalism also occurs; e. g.
Polynoe Scolopendrina and Harmothoe Marphysae inhabit the tunnels of Marphysa sanguinea.
The fixed Tubicola with calcareous tubes are found from the Silurian upwards. Undoubted remains of Errantia occur in the Solenhofen slates, and probably in the Silurian (Nereites, Crossopodid). Structures from the
1For the rosette organs of Tomopteridae, which consist of an assemblage of yellow coloured sacs with a nerve supply, and are apparently phosphorescent organs, see Greeff, Z. W. Z. xlii. p. 440; and on the elytra of Polynoe which are phosphorescent, Jourdan, Z. A. viii. 1885.
2 The Polychaeta live at all depths in the ocean. 'No definite law as to the presence or absence of genera at particular depths can be enunciated,' but 'the majority of abyssal forms are tube-dwellers' (Mcintosh). A Terebellid Leaena abyssorum and a Serpulid Placostegus benthalianus were dredged by the Challenger in 3212 fathoms. Cf. Challenger Reports, xii. 1885.
3 Fritz Mullcr, Facts for Darwin, 1869, p. 44.
Silurian, Devonian and Carboniferous have been identified as Eunicidan jaws.
The class Chaetopoda contains, in addition to the two orders Polyehaeta and Oligochaeta, two others, the Chaetopoda ectoparasitica and the Archi-Chaetopoda, described separately here, because of their interesting peculiarities. The first-named order contains highly modified worms, but its position among Chaetopoda must be regarded as settled by the discovery of the larval form. The Archi-Chaetopoda resemble the Archi-Annelida (infra, p. 613) in the large size of the peri-stomial somite, the similarity inter se of the remaining somites, and in the position of the nervous system within the hypodermis, as well as in the connection of the intra-muscular nervous plexus with cells of the hypodermis.
There are two genera included in this group - Myzostoma, parasitic on various Comatulidae and a few Pentacrinoids, and Stelechopus, parasitic on Hyocrinus sp. ? The body of Myzostoma is symmetrical and non-segmented, usually disc-shaped, occasionally elongate. It is provided with five pairs of short ventral and radially arranged parapodia, each armed with a chitinoid hook and a supporting rod; five pairs of suckers commonly alternate in position with the parapodia. The margin of the body has ten pairs of cirri at least, or a larger number, which are furnished with terminal bundles of stiff sensory (?) setae and a ventral furrow lined by adhesive cells. There is a chitinoid cuticle, and cilia occur in isolated patches on the dorsal and ventral surfaces. The nervous system consists of a large ventral ganglion, with five pairs of lateral nerves. A supra-oesophageal ganglion appears to be absent. The alimentary canal consists of a protrusible muscular proboscis, an oesophagus, a stomach, which gives off lateral branched caeca, and a short rectum. Mouth and anus are generally on the ventral surface, the former anterior, the latter posterior in position. There are no nephridia. The majority are hermaphrodite.
The testes are ventral, paired, and branched except in the dioecious forms, and the male apertures are lateral, and placed externally to the third pair of parapodia. The ovaries are represented by a number of caeca on the dorsal surface; an oviducal tube opens into the dorsal aspect of the rectum. M. cysticolum has rudimentary testes; M. tenuispinum, inftator, Murrayi, are dioecious, i.e. either male or female. M. glabrum, though hermaphrodite, has small 'complemental' males, like certain hermaphrodite Cirri-pedia (p. 537). M. glabrum and cirriferum have larvae provided with a post-oral and a prae-anal band of cilia, with a larval supra-oesophageal ganglion, and two bundles of provisional setae; they closely resemble the larval Nereis Dumerilii. The position of the Myzostomidae in the class Chaetopoda must accordingly be regarded as a settled fact.
Stelechopus differs from Myzostoma in being elongate, in the parapodia not being radially arranged, in the absence of suckers and cirri, and in the absence of lateral caeca to its alimentary canal; ? hermaphrodite.
Von Graff, Challenger Reports, x. 1884; Id. Das Genus Myzostoma, Leipzig, 1877. Suckers, Niemiec, Recueil Zool. Suisse, ii. 1885. Development, Beard, Mitth. Zool. Stat. Naples, v. 1884.