Functional morphology of brachiopod and coral skeletal structures supporting ciliated epithelia

Ciliary transport of particulate suspensions in water was studied in the mantles of the inar- ticulate brachiopod Lingula anatina, the articulate brachiopod Coptothyris grayi and in the ciliated epithelium of the scleractinian coral Montipora foliosa. In both brachiopod species a close correlation was found between the water currents induced by the lophophore and by the mantle. Lophophore and mantle are considered here as functionally integrated organs which are active in the transport and capture of food, accumulation and the removal of (pseudo-)faeces, the sorting of particles, the pumping of water and respiration. The tuberculate surface of Montipora foliosa is lined with ciliated epithelium; the polyps are situated in the depressions of this surface. The particulate suspension within the labyrinth is constantly diluted: particles are funneled through the furrows but end up at the tips of the tubercles. On their way they pick up some mucus. While they concentrate on the tubercles they coalesce to form larger boluses that are easily removed. The tubercular tips thus act as "bolus accumulators" and the unciliated margins of the brachiopod mantles appear to have the same function. In this accumulated form the particles are probably prevented from being recycled through the food-capturing system. Based on these observations a preliminary functional interpretation is presented of some fossil bra- chiopods: the Devonian rhynchonellids Uncinulus and Hypothyridina and the Strophomenida, with the genus Marginifera serving as an example of the latter order. The morphology of the inner shell surface, and the inferred configuration of the lining ciliated mantle, was modified in these taxa by a variety of structures such as spines, tubercles, diaphragms and trails. It is suggested that the mantle of the evolved Uncinulidae and of the Strophomenida is particularly suited for the dilution of particulate suspensions before they reach the lophophore and for the accumulation and removal of mucous pseudofaeces. The inhibitory effect of the constrictions on the circulation of water could be counteracted by local ciliary pumping. In the strophomenids the mantle became a specialized and elaborate organ, while the lopho- phore probably remained relatively simple. In the other articulate stocks the lophophore became the central food-gathering and pumping device and in general the morphology of the mantle was maintained simple.