PELECYPODS 



411 



1, Solecurtus strigillatut, showing partly united siphons covered with leathery integument:*, af., 

 incurrent siphon ; . ef. , excurrent siphon; SS, the two uniting. 2, Cardium edvle, showing partly 

 united siphons with papillaceous orifices : A, excurrent ; B, incurrent siphon ; F, foot. 3, Scrolricu- 

 laria piperata, in its natural position in the sand, showing very long tubular siphons . A, excur- 

 rent siphon ; B, incurrent siphon. 



the mantle lobes unite in order to form two openings, 

 openings are called the siphons. 



These 



SIPHONS 



In some cases the mantle lobes do not actually unite to form 

 regular siphonal openings, but in life the free mantle-edges have 

 a way of adjusting themselves posteriorly to form functional 

 siphons without actually coalescing. Usually, however, the man- 

 tle flaps not only unite posteriorly to form true siphons, but are 

 capable at that point of varying degrees of protrusion from the 

 shell, and when extended the siphons appear as two tubes. In 

 some genera these siphonal tubes are very long ; in others they 

 are fastened together and surrounded by a tough, leathery integu- 



