396 



MANUAL OF THE MOLLUSC A. 



niently examined in a bivalve with a' closed mantle, like the 



great Mya (Fig. 207), wliicli 

 lives in the mud of tidal rivers, 

 ■with only the ends of its long 

 combined siphons exposed at the 

 surface.* The siphons can be ex- 

 tended twice the length of the 

 shell, or drawn completely with- 

 in it ; they are separated, in- 

 ternally, by a thick muscular 

 wall. The branchial siphon (s) 

 has its orifice surrounded by 

 a double fringe ; the exhalent 

 siphon [s) has but a single row 

 of tentacles ; these organs are 

 very sensitive, and if rudely 

 touched the orifices close and 

 the siphon itseK is rapidly with- 

 drawn. When unmolested, a cur- 

 rent flows steadily into the orifice 

 of the branchial siphon, whilst 

 another current rises up from 

 the exhalent tube. There is no 

 other opening in the mantle ex- 

 cept a small slit in front {p) 

 through which the foot is pro- 

 truded. The body of the animal 

 occupies the centre of the shell 

 (6), and in front of it is the 

 mouth (o) furnished with an 

 upper and a lower lip, which are 

 f prolonged on each side into a 

 pair of largo membranous palpi 

 [t). The gills {(]) are placed two 

 on each side of the body, and 

 are attached along their upper, 



or dorsal margins ; behind the body they are united to each other 



Fi^. 207. Mya arenaria. t 



* Alder and Hancock on the branchial currents of Pholm and Mya. An. Nat. 

 Hist.. Nov. 18.'>1. 



t Mya arennritty L. (original, from specimens obtained at Sonthend, and commu- 

 nicated by Miss Hume). Tlie left valve and mantle lobe and half the siphons are re- 

 moved, rt', c', adductor muscles ; ft, body ; c, cloaca ;/, foot ; gr, branchiae ; A, heart; 

 m, cut edge of the mantle ; o, mouth ; s, s\ siphons ; t, labial tentacles ; lu vent. The 

 arrows indicate llie direction of the currents ; the four rows of dots at the base of the 

 gills are the orifices of the branchial tubes, opening into the dorsal channels. 



