摘要：

contraction of the lateral body muscle layer produces a hydrostatic pressure on the body's coelomic cavity; the body volume is shifted posteriorly and laterally; the valves gape about 6° to rest against the lateral bur- row walls, which act as supports; and the lo- phophore extends to become functional within an enlarged pallial cavity. The normal life position is static and can be maintained without much effort. The pedicle plays no role in the maintenance of this position. Occasional scissorlike movements of the valves assist in maintenance of the burrow (Fig. 409).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 m) higher than 35 to 40 percent because in such

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