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. [Illustration: 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] _Tardigrada._