Of the Annelida the only orders which are known to have left any traces of their existence in past time are the Tubicola and the Errantia; of which the former are known by their investing tubes, whilst the latter are recognised by the tracks which they left upon ancient sea-bottoms, or by their burrows in sand or mud, or, more satisfactorily, by their horny jaws, remains of this latter nature being known from deposits as old as the Lower Silurian.
Tubicolar Annelides are known to occur from the Silurian rocks upwards. The well-known Silurian fossil Tentaculites, has been often referred to the Tubicola, but is almost certainly Pteropodous. Cornulites. Serpulites, Ortonia, Trachyderma, Spirorbis, and Conchicolites are, however, genuine Silurian Tubicola. The Microconchus carbonarius is a little spiral Tubicolar Annelide, nearly allied to the Spirorbis (fig. 130, b) of our seas, which is not uncommonly found in strata belonging to the Carboniferous period; and the genus Spirorbis itself is represented even in the Silurian period. Class III. Chaetognatha (Huxley). - Elongated cylindrical animals having the hinder extremity of the body furnished with an integumentary fin. Anterior end of the body provided with setae. and corneous jaws. No foot-tubercles. Sexes united in the same individual.
This class includes only the singular pelagic animals belonging to the genus Sagitta, the precise systematic position of which is somewhat doubtful. They appear, however, to form a connecting link between the Annelides on the one hand, and the free Nematoids on the other hand.
The Sagittae (fig. 134) have elongated transparent bodies, rarely over an inch in length, having the hinder end of the body expanded into a striated caudal fin, similar fins often existing on the sides of the body as well. The head carries a series of setae placed in front of the mouth, and the oral aperture is furnished with unciform corneous bristles or "falces," which act as jaws. The alimentary canal is straight, and terminates in an anus placed at the base of the tail below.
"A single oval ganglion lies in the abdomen, and sends, forwards and backwards, two pairs of lateral cords. The lateral cords unite in front of and above the mouth into a hexagonal ganglion. This gives off two branches which dilate at their extremities into the spheroidal ganglia, on which the darkly pigmented imperfect eyes rest. The ovaries, saccular organs, lie on each side of the intestine and open on either side of the vent; receptacula seminis are present. Behind the anus, the cavity of the tapering caudal part of the body is partitioned into two compartments; on the lateral parietes of these, cellular masses are developed which become detached, and, floating freely in the compartment, develop into spermatozoa. These escape by spout-like lateral ducts, the dilated bases of which perform the part of vesiculae seminales. The embryos are not ciliated, and undergo no metamorphosis" (Huxley).
The species of Sagitta are found, living in the open sea, in the Mediterranean, and in the Atlantic and Pacific Oceans.
Fig. 134. - Morphology of Chaetognatha. A, Sagitta tricuspidata, of the natural size: o One of the ovaries; sp Orifice of one of male organs of reproduction. B, Head of the same, viewed from beneath and greatly enlarged, showing the horny, setiform jaws. (After Saville Kent.)