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 

 r,o 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 NTJCULIDJE 



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 Nuculldce 

 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 cronulated or 

 scalloped, and there is n.o trace of a byssus. The shells are equi- 



