PELECYPODS 411 
a—=- 
---> 
1, Solecurtus strigillatus, showing partly united siphons covered with leathery integument: s. af., 
incurrent siphon ; s.ef., excurrent siphon; SS, the two uniting. 2, Cardium edule, showing partly 
united siphons with papillaceous orifices: 4, excurrent; B, incurrent siphon; F, foot. 3, Scrobicu- 
laria piperata, in its natural position in the sand, showing very long tubular siphons: 4, excur- 
rent siphon; B, incurrent siphon. 
the mantle lobes unite in order to form two openings. These 
openings are called the siphons. 
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- 
