Coarser or muddier substrates are less well
suited for providing stable burrow walls.
Consequently, the animal is unable to live or
to survive in sediment that is too coarse or
too muddy, contrary to general assumptions
about habitats of lingulides. Thus living
lingulides have rarely been found in muddy
sediments with a fine fraction (< 63 mm)
higher than 35 to 40 percent because in such
fluid sediments the walls, even when bonded
by mucous secretion, inadequately support
the shell in its normal filtering position
(EMIG, 1983a).
At the surface of the sediment, three characteristic
pseudosiphons indicate the presence
of a lingulide in normal, life position
(Fig. 407–408, 410; Table 35). They are
shaped by the highly specialized anterior setae
of the mantle. At the level of the shortest
setae, the anterior mantle margin of each
valve develops an epidermal crest. These
come into contact with each other and induce
tilting and interlacing of the setae
borne by the crests. Simultaneously the longest
setae, which can be as long as a third of
the shell length, remain vertical (Fig. 408).
The central aperture is exhalant, while the
two lateral apertures are inhalant. The exhalant
and inhalant water streams are completely
separated by the mantle crests and
internally by tentacle tips without any mixing
of the flows.