Annelida.—This name is given to a class of Articulata, consisting of worms, whose bodies are formed of little rings, or annular segments, and which have red blood; as the Leech, Earth-worm, &c. Some are naked (the Dorsibranchiata and Abranchiata), and move with great celerity; as the Gordius, or Hair-worm, and the Nereis, so frequent on the sands of the sea-shore. Others have shelly coverings (the Tubicolæ), as the Serpula, and are sedentary, or fixed to other bodies. The soft bodies of certain species are protected by a coat, or tube, formed by the agglutination of sand, or other foreign substances, as in the Sabella ([Lign. 123], fig. 6, p. 385).
The fossil remains of the testaceous Annelides are very abundant in some deposits; and even the naked, flexible, soft-bodied forms have left proofs of their existence in some of the most ancient sedimentary rocks. Traces of nine species, belonging to five genera of these soft, naked Annelides, have been observed in the Silurian strata of Britain.
Lign. 166. Fossil impression of Nereis. Silurian strata.
(Drawn by Miss Murray.)
Nereites Cambrensis. (Murch. Sil. Syst.) Llampeter.
FOSSIL NEREIS. SERPULA. CIRRIPEDIA.
The first notice of these remarkable remains appeared in the invaluable work of Sir R. I. Murchison on the Silurian System.[430] The living species of Nereis (Dorsibranchiate) are free, agile animals, having a distinct head, provided with either eyes or antennæ, or both; they are the most perfect in structure of all the Annelides. The fossil represented in [Lign. 166] indicates that the body of the original was composed of about one hundred and twenty segments; the feet were half the length of a segment of the body; and the cirri of the feet were longer than such segment. A more slender species, (Nereites Sedgwickii,) the body consisting of a greater number of segments, is also figured and described by Sir It. I. Murchison. Other impressions in the same stone resemble those that would be produced by smooth Annelides (Abranchiate) related to the Gordius, or Hair-worm.[431]
[430] Murch. Sil. Syst. p. 699.
[431] Murch. Sil. Syst. p. 701, pl. xxvii.; and M’Coy, Cambridge Pal. Foss. p. 128, pl. 1. D.
Serpula.—The animals of this genus are sedentary or fixed, having calcareous tubes or shells, but to which they have no muscular attachment. They have plumose or arborescent gills affixed to the anterior part of the body. The shelly tubes of the Serpulæ are constantly seen on our coasts, encrusting stones, rocks, shells, sea-weeds, &c., and may be known by their contorted or twisted forms. There are a hundred and fifty British fossil Tubicolæ. A large species has been discovered in the Silurian rocks (Murch. Sil. Syst. pl. v. fig. 1); several occur in the Carboniferous, Oolitic, and Cretaceous, and many in the Tertiary strata. In the Upper Chalk, a smooth tortuous Serpula is not uncommon (S. plexus, Min. Conch, tab. 598); it occurs in masses several inches long. But I have not observed either in the Chalk, or in any other deposit, indications of banks of Serpulidæ, like those now in progress off the Bermudas, and which resemble coral-reefs in their solidity and extent.