PALLIAL prolongations. 
•200 
of foniiiiiK the pallial margin of the respiratory orifice into a long, 
conical and extensile tube, the delicately sensitive apical orifice of 
which is kept closed until it reaches the surface of the water, when 
it is opened to the air for respiratory purposes. 
In the Pelecypoda the develoiinient of the branchial and anal 
orifices to form elongate siphons is ai)parently one of the modifica- 
tions more esi)ecially fitting the mollusk for flnviatile or estuarine life 
hut is more palpably correlated with the habit of burrowing deejily 
in the muddy or sandy bed of the water they inhabit, and where this 
lieculiar habit of concealment is carried out to the greatest extent 
the longest siidions are developed, and the constant and regular 
stream of water necessary for respiratory and alimentary purposes is 
only able to enter and leave the pallial cavity by means of the 
elongate si])hons, whose protractile orifices i)roject into the water above 
tlie place of concealment of the animal ; while in those species which 
Fig. 395. — Pisuiiunt amnicttin (Miill.) Fig. 396. — Spha'rium corneum (L.) X 2, 
X 2, illustraling the development of a single illustrating the development of two distinct 
siphon. siphonal tubes. 
Kettering, collected by Mr. C. E. Wright. Kettering, collected by Mr. C. E. Wright. 
live or move freely in the water, the mantle is usually completely open 
with res})iratory apertures very slightly or not at all prolonged and the 
papillcC, tentacles and other protuberances more or less sensory in 
character are distributed indiscriminately along all the free mantle 
margin. In tlie burrowing forms these accessory sensoiy organs are 
more or less concentrated at the posterior margin, around the siphonal 
ai)ertures, but more especially at the branchial or inhalent opening. 
These prolonged muscular tubes or siphons, which are mostly 
protruded by blood pressure and withdrawn by the contraction of the 
modified orbicular muscles, are often separated and form two out- 
wardly distinct channels, as in Sphwnum, but are very variable in 
their relative length; in Dreissensia and S}}hwrium the branchial 
siphon is longer than the anal one ; but in Pisidimi the reverse is very 
strikingly shown, as although the anal siphon is greatly prolonged, 
the branchial aperture remains unseparated and undifferentiated from 
the great mantle cleft. 
