896 



MANUAL OF THE MOLLITSCA. 



niently exairincd in a bivalve with a closed mantle, like tlie 



great Mya (Fig. 207), which 

 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 dra-s^m 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 sii^hon itself is rapidly with- 

 drawn. Whenunmolested, 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 ]Dro- 

 truded. The body of the animal 

 occupies the centre of the shell 

 (&), and in front of it is the 

 mouth (o) furnished with an 

 upi^er and a lower lip, which are 

 f prolonged on each side into a 

 pair of large membranous palpi 

 {t). The gills [g) are-'iDlaced two 

 on each side of the body, and 

 are attached along their upper, 



ripf. 207. M'ja are.naria. t 

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



* Alder and Hancock on the branchial cuiTents of Pholas and Mya. An. Nat. 

 Hist., Nov. 1851. 



t Mya arenaria, L. (original, from specimens obtained at Southend, and commu- 

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

 moved, ff, a', adductor muscles ; J, body ; c, cloaca ;/, foot ; gr, branchiae ; A, heart ; 

 m, cut edge of the mantle ; o, mouth ; s, s', siphons ; t, labial tentacles ; v, vent. The 

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

 giils nre tha orifice-; of the branchial tubes, opening into the dorsal channsk. 



