PELECYPODS 439 
by connective tissue, which sometimes is dense enough (though 
always porous) to obscure the reticulated appearance of the 
organ. Furthermore, the edge of the outer lamella of the outer 
gill unites with the mantle; likewise the edge of the inner lamella 
of the inner gill unites with the body-mass, and behind the body- 
mass with the gills upon the other side of the animal. This 
union of the four gills in a straight median line behind the foot 
and body-mass forms a septum, which divides off from the mantle 
cavity a “suprabranchial” cavity. Into this cavity the excreta 
are discharged, and with it the anal or excurrent siphon con- 
nects. The branchial or incurrent siphon thus pours its current 
of water into the lower mantle cavity, to bathe the gills and feed 
the mouth, while the fresh water is not contaminated by any 
open and wide connection with the excretory processes. In 
many of the forms where the siphons are very long and united, 
this septum, formed by the union of the dorsal edges of the inner 
gills back of the foot, is continued far into the siphons, consti- 
tuting the division of the two into the anal and branchial 
siphons. 
Another feature of this order is the marked tendency to man- 
tle fusion. As there are always siphons, the mantle-edges must 
necessarily have fused at two points at least, in order to have 
formed the siphons. The tendency, however, is for the fusion to 
continue, narrowing the pedal opening more and more, as though 
the mantle were endeavoring to envelop the entire animal in a 
bag or sac open only at one end (siphonal opening), but yet 
slightly ripped along one seam (pedal opening). 
This order includes the great majority of pelecypods, and admits 
of many very confusing modifications in its most essential features. 
FaMILY CARDITIDE 
In this, the first family of the order, most of the characteristic 
features just spoken of are present. Mantle fusion, however, has 
not progressed beyond a degree which is just sufficient to form 
the siphons. There is a byssiferous gland. The name Carditide 
refers to the heart-shape of the shells. 
