396 MANUAL OF THE MOLLUSCA. 
niently examined in a bivalye with a closed mantle, like the 
Fic. 207. Mya arenaria. ¢ 
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 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 itself is rapidly with- 
drawn. Whenunmolested, acur- 
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 sht in front (p) 
through which the foot is pro- 
truded. The body of the animal 
occupies the centre of the shell 
(b), and in front of it is the 
mouth (0) furnished with an 
upper and a lower lip, which are 
-/ prolonged on each side into a 
pair of large membranous palpi 
(t). The gills (7) 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 
* Alder and Hancock on the branchial currents of Pholas and Mya. An. Nat. 
Hist., Nov. 1851. 
4 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; b, body; c, cloaca; f, foot; g, branchie; h, heart; 
m, cut edge of the mantle; 0, mouth; s, s’, siphons; ¢, 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. 
