The rays have the mouth, nostrils, and gill-openings, on the under-side. Like the sharks, they are without gill-covers, and have the gills fixed by both margins. Their teeth differ from those of all ordinary
Fig. 2.—Sting-Ray {Trygon hastata, Storer).
fishes, being of a definite form, bounded by planes symmetrically arranged, the whole forming a beautiful mosaic (Fig. 3).
Remarkable as are the form and general structure of the rays, as indicated above, a still more remarkable structure is exhibited in some
Fig. 3.—Teeth of a Ray.
of them; for those known as torpedoes (Fig. 4) are so constructed that they are a powerful galvanic battery. These have the space between the pectoral fins, the head, and the gills, on each side, filled with membranous tubes which are divided by horizontal partitions into small cells filled with a sort of mucus and traversed by nerves;
