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 CARDITIDJE 



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 Carditidce 

 refers to the heart-shape of the shells. 



