164 



POSITION OF BRANCHIAE IN PELECYPODA 



the temperature 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 Pehcypoda, 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, elongated, 



Fig. 72. — Cardium 

 edule L.: A, anal ; 

 B.branchial siphon ; 

 F, foot. (After 

 Mobius.) 



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. — Scrohicidaria piperata Gmel., in its natural position in tlie sand: A, efferent 

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



chamber known as the hranchial cavity. Leading into this 

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

 which conducts water to the branchiae, while the other carries it 

 away after it has passed over them. The lower is known as the 

 hrancJiial or afferent siphon, the upper as the arial or efferent 

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

 can readily be observed by placing a little carmine in water, 



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



