THE LAMELLIBRANCHIA 209 
exhalant in this genus ; but is posterior and only exhalant in Lutravia, 
Thracia, ete. 
Frequently the two posterior pallial orifices, anal and branchial, 
or at least the anal, as in some Lucinidae, are more or less pro- 
longed in the form of muscular tubes which may be extended for a 
greater or less distance beyond the shell (Fig. 190); this feature is 
found in the majority of burrowing and boring Lamellibranchs. 
These tubes are known as siphons, and may be eitler—(1) inde- 
pendent of one another throughout their length, as, for example, in 
the Tellinidae (Figs. 190, b7.s, a.s; 245, g, g’), Donacidae, Thracia, 
ete.; or (2) partially fused together, as may be seen in J apes, 
Solenocurtus (Fig. 194), Saxicava (Fig. 246); or (3) completely 
united to one another, as in Mactra, Dosinia, Mya. Lutraria, Pholas, 
Teredo (Fig. 195), ete. The branchial siphon is usually the longer 
of the two, and in Scrobicularia is more than four times as long as 
the body. Sometimes the siphons may attain to a still greater 
=> Wy, “ee 
Fic. 190. 
Tellina planata, left-side view. a.s, anal siphon; br.s, branchial siphon; jf, foot; pa, 
tentaculiferous border of the mantle; sh, shell. (After Poli.) 
degree of development, and may form an important part of the 
body-mass, or even surpass it in size. The extreme limit of evolu- 
tion in this direction is exhibited by Teredo, in which these organs, 
while receiving the same nerve supply as in other Lamellibranchia, 
constitute the greater part of the mass of the animal; the anal 
siphon extends right forward and the branchial siphon contains the 
gills, as is also the case in Solenocurtus, Fistulana, Saxicava, ete. 
There are two opposite currents in the pallial cavity. The first 
is postero-anterior in direction, and is set up by the action of the 
cilia clothing the surface of the branchiae ; the second runs in the 
reverse direction on the ventral side near the edges of the mantle, 
and wards off foreign substances from the pallial cavity (Jytilus, 
Cardium, etc.). In Pinna and Solen this second current is created 
by a large longitudinal ciliated ridge on each side. 
The two lobes of the mantle are furnished with several distinct 
pallial muscles which are inserted on the shell and are divisible into 
the following distinct groups :—(1) The orbicular muscle, extending 
right round the periphery of each lobe, and serving to attach it to 
14 
