Along the shores of the Channel and in the Mediterranean there are few bivalves more abundant than the several species of the genus Donax. They live near the shore in shallow water, burying themselves perpendicularly in the sand. They have the very singular habit, considering their apparent helplessness, of being able to leap to a certain height and then project themselves ten or twelve inches. This may often be witnessed in the case of individuals left by the retreating tide. If seized by the hand, and attempts are made to disengage them from the sand, they continue to impress on their shell a sudden and energetic movement, aided by the elasticity of their foot, which is at once decisive and angular.
Fig. 136. Donax rugosus (Linnæus).
Fig. 137. Donax denticulatus.
The shell of the Donax is nearly triangular in shape, compressed, longer than it is high, regular, equivalve, not equilateral; the hinge with three or four teeth on each valve.
The animal is slightly compressed, and more or less triangular. Its mantle, which forms two symmetrical lobes enveloping the body, is open pretty nearly in all its extent, but it is united posteriorly, and terminates in two syphons or nearly equal tubes, as in Fig. 130, p. 326. One of these tubes serves the purpose of respiration: it is the bronchial syphon. The other, serving the purpose of ejecting the products of digestion, is termed the anal tube. The tentacles of the bronchial tube seem to be possessed of exquisite sensibility. When touched, the animal draws in its syphon, and only puts it forth anew when the danger has passed. The species of Donax are very numerous, especially in the Asiatic and American seas. Among the European species we may mention Donax rugosus (Fig. 136) and Donax denticulatus (Fig. 137).
Fig. 138. Tellina radiata (Linnæus).
