896 



MANITAL OF THE MOLLUSCA. 



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



great My a (Fig. 207), which 

 lives in the mnd 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. 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 {jp) 

 through which the foot is pro- 

 truded. The body of the animal 

 occupies the centre of the shell 

 (5), 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 large membranous palpi 

 [t). ThegiLls [g] are placed two 



on each side of the body, and 



ripr. 207. Mya arenaria. t j_i t t ^ ±t • 



are attached along their upper, 



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



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

 Hist., Nov. 1851. 



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

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

 moved, a, a', adductor muscles ; 6, body ; c, cloaca ;/, foot ; gr, branchiae ; A, heart; 

 m, cut edge of the mantle ; o, mouth ; 5, s', siphons ; t, labial tentacles ; v, vent. The 

 arrows indicate Ihe 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. 



