BIVALVE MOLLUSC A. 



337 



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 siphons, or nearly equal tubes, as in Fig. 127, 



Fig. 137. Tellina radiata (Linnaeus). 



p. 320. One of these tubes serves the purpose of respiration: it is 

 the respiratory siphon. The other, serving the purpose of ejecting 

 the products of digestion and the used-up water, is termed the ex- 

 current siphon. The tentacles of the respiratory siphon seem to be 



Fig. 138. Tellina virgata (Linnaeus). 



Fig. 139. Tellina sulphurea (Lamarck). 



possessed of exquisite sensibility. When touched, the animal draws 

 in its siphon, 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. 135) and Donax denticnlatus (Fig. 136). 



