Every hair, cirrus, and tentacle on the bodies of the Errant Worms is a living organ of feeling, shrinking at the smallest touch, but enabling them to select their food, to move towards and retreat from objects, and to thread their way through the most intricate labyrinths with unerring certainty, which seems to render them independent of eyes; yet many of them have multitudes of eyes, or rather eye-specks, according to the genera. Some have but one eye-speck placed in the forehead; one genus has a double row throughout their whole length, two in each segment, while the Amphicora has two in its tail. All these eye-specks have their crystalline lens, pigment-layer, and nerve-bulb, so that the Errant Worms must see objects, and their motions show that they do; but we can form no idea of the kind of vision.
Besides the variety of organs on the skin of the Errantia, some of these worms have two rows of flat plates on their backs overlapping each other at their edges like the scales of a fish. They are well developed in the Aphrodita hystrix, or the Sea Mouse of fishermen, and its congeners. That Annelid, which is an inhabitant of European coasts, is thicker and broader than other sea-worms. The two rows of overlapping shields on its back, and the quantity of iridescent hairs, cirri, and other appendages covering the body, is so great as to form a kind of felt or fur like the skin of a mouse. The members of this genus of sea-worms have no gills properly so called; the only external sign of respiration is a periodical elevation and depression of the shields on their backs by the action of a complex system of muscles. The thick covering of felt on the body of the worm below the shields becomes filled with water during their elevation, which is ejected forcibly at the posterior end of the body during their depression. Although the water does not penetrate the thin skin on the back of the worm, its oxygen does, and is accumulated in the colourless liquid in which the stomach floats; and from it the blood, which is of a pale yellow colour, receives its oxygen. The feet of the worm are fan-shaped groups of sharp glassy bristles enclosed between two plates, which prevent them from hurting the animal when it puts them out or draws them in. The Aphrodita is male and female: the eggs escape through pores in the female, and are received in a kind of pouch beneath the dorsal shields till hatched. The embryo is an oval locomotive mass, with groups of cilia, and indications of an eye-speck: after swimming about for twenty-four hours, the segments begin to be developed.
Worms of the genus Polynoë have also two rows of shields on their backs, but they are studded with transparent oval bodies on short stems, supposed to be organs of touch. The filiform tentacles and antennæ that are developed between the shields, as well as the cirri or curly bristles of the feet, are likewise covered with similar sensitive organs. [Fig. 135] shows the foot, cirri, and bristles of a Polynoë, which are enclosed in plates which preserve them from hurting the worm. These glassy bristles are beautiful objects under the microscope; still more so are the jointed feet, transparent as the purest flint glass, of the Phyllodoce viridis, one of the most beautiful Annelids on our coasts, where it threads its way among young mollusca like a slender green cord, exhibiting foliaceous gills in the highest perfection.
Fig. 135. Foot of a Polynoë.
In the marine Annelids the embryo, on leaving the egg, is a gelatinous globular mass of cells furnished with strong cilia. In a few hours the mass elongates and divides into four parts, a head, a large ciliated segment, a smaller one without cilia, and a ciliated tail. After a time a succession of new segments are interposed, one by one, next to the tail segment, and the corresponding internal organs of each are developed till the worm arrives at its adult state. In many Annelids the embryo is highly developed within the parent; that of the Eunice has from 100 to 120 segments before it leaves her; and in the Nereis diversicolor the young, covered with cilia, come out by hundreds at an orifice in the side of the mother.
Many of the marine Annelids are luminous; electric scintillations are given out during the act of nervous contraction, which are increased in brilliancy and rapidity by irritation.
According to Professeur Quatrefages, the Annelida Errantia and Tubicola have no zoological regions characterized by one or more special types like the other classes of animals; they have representatives in all seas. But it is exactly the contrary with regard to species. The number of species common to any two seas, or the shores of two continents, is very small; there is not a single species common to the Atlantic coasts of France and the Mediterranean. The sea-worms are not affected by climate, but they are said to be more abundant on granitic and schistose coasts than on the calcareous.[[34]]
With regard to fossil remains, worm-tracks are seen in the Forest marble, long calcareous tubes occur in the Upper Silurian and Carboniferous strata, and in all the later formations tubercular Annelids abound, especially of the genera Serpula, Spirorbis, and Vermilia.[[35]]