PELECYPODS 421 
developed. The adjacent filaments are connected, not by cilia, 
but by vascular channels, and the depending and recurved por- 
tions of each filament are also connected in a manner presumed 
to admit of the passage of blood. This gives to the organ the 
appearance of basketwork or network. The ends of the filaments 
are attached to the mantle and to the visceral mass, as in the 
last order, forming cavities or chambers above (see Fig. C). This 
order includes the great majority of the Pelecypoda. 
The fifth and last order is confined to but two rather obscure 
families, in which the gill-development is carried to the point of 
substitution of muscular partitions which form a separate cham- 
ber connected with the mantle cavity by a narrow slit, the surface 
of the chamber having respiratory functions (Fig. D). 
The collector of living marine objects who has become fasci- 
nated by their beauty and who derives pleasure from examining 
the greater beauty of their structures will find a rich field for 
research in the pelecypod gills. But unless he has time at his 
disposal and some skill with a microscope, he would do well not 
to venture too far into this alluring but difficult and vexatious 
subject. 
Our coastal waters are fairly rich in bivalves, but the most 
strikingly handsome species of this class—and there are many of 
them—are inhabitants of warmer seas. Of the great number 
of bivalves to be found upon our own shores we can only men- 
tion briefly some of the commonest species. 
ORDER PROTOBRANCHIATA 
FAMILY NUCULIDE 
In this family the gills are of the simplest and most primitive 
type. The mantle-edges are entirely open along the ventral 
margin, but they are united posteriorly in some of the Nuculide 
to form two short siphons. The palps are exceedingly large. The 
foot is also large, and by its disk-like surface is adapted to creep- 
ing, much as is a gasteropod foot ; its edges are often crenulated or 
scalloped, and there is no trace of a byssus. The shells are equi- 
