412 MARINE INVERTEBRATES 
ment, which, like the siphons proper, is only an extended portion 
of the mantle-edge. The orifices of the siphons are generally 
papillaceous. These two siphons (for there are always two if 
there are any at all) are the anal or excurrent (upper) and the 
branchial or incurrent siphon. The function of the latter is to 
draw in the pure water to bathe the gills and to furnish food, 
while the office of the former is to eject waste materials and the 
water which has already passed over the gills. 
Diagram illustrating the various degrees of union of the mantle lobes: A, mantle completely 
open; B, rudiments of siphons, mantle still completely open; C, mantle closed at one point; D, 
mantle closed at two points, with complete formation of siphonal apertures; E, development of 
siphons, ventral closure more extended; F, mantle closed at three points, with fourth orifice; f, 
foot; s.a., 8. b., anal and branchial siphons; 1, 2, 3, first, second, and third points of closure of 
mantle; b.0., byssal aperture. 
MANTLE FUSION 
Besides the points of juncture where the mantle-edges unite 
to form the siphons, the coalescence of the two mantle lobes may 
be extended, and they may become further united and fused toge- 
ther at other points. Indeed, the fusion of the mantle-edges may 
become almost complete; but it always leaves the siphonal open- 
ings and a third opening through which the foot may be extended. 
This opening for the foot is called the pedal opening, or orifice. 
In such cases, therefore, the mantle becomes much like a bag or 
sac. The degree of fusion of the mantle-edges appears to corre- 
spond closely with certain changes in the organization of the ani- 
mal, and this feature must be regarded as a very important 
character. 
