164 



POSITION OF BRANCHIAE IN PELECYPODA 



CHAP- 



temperature one or two degrees C, but as a rule, the temper- 

 ature of a solitary Helix differs very slightly from that of the 

 surrounding air. Increased activity, whether in respiration or 

 feeding, is found to raise the temperature. 



W. H. Dall, writing of the branchia in Pelecypoda^ remarks^ 

 that there can be no doubt that its original form was a simple 

 pinched-up lamella or fold of the skin or mantle. This, elon- 



FiG. 72. — Cardium 

 edule L. : A, anal ; 

 B, branchial siph- 

 on; F, foot. (After 

 Mobius.) 



gated, becomes a filament. Filaments united by suitable tissue, 

 trussed, propped, and stayed by a chitinous skeleton, result in 

 the forms, wonderful in number and complexity, which puzzle 

 the student to describe, much more to classify. 



In Pelecypoda the branchiae are placed on each side of the 

 body, between the mantle and the visceral mass. They lie in a 



Fig. 73. — Scrobicularia piperata Gmel., in its natnral position in the sand: A, effer- 

 ent or anal siphon ; B, afferent or branchial siphon. (After Mobius.) 



chamber known as the hrancJiial cavity. Leading into this 

 cavity, and behind it, are, as a rule, two tubes or siphons, one 

 of which conducts Avater to the branchiae, while the other 

 carries it away after it has passed over them. The lower is 

 known as the hraneJiial or afferent siphon, the upper as the anal 

 or efferent siphon (see Figs. 72 and 73). The action of these 

 siphons can readily be observed by placing a little carmine in 



1 Bull. Mus. C. Z. Harv. xviii. p. 434, 



