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A number of ascidians form mineralized spicules at the post-larval stage. Depending on the species, the spicules are located in one or several of the following body sites: test, body wall, branchial sac, gonads, or still other tissues. Spicules of several species of the stolidobranch
genera Bathypera, Pyura, and Culeolus in the family Pyuridae were examined in this study and compared with those of Herdmania published earlier. Scanning electron microscopy shows much variability in spicular morphology at the genus level. Each genus except for Pyura
has spicules of only one basic building plan, whereas in Pyura they not only usually differ between species, but within a species differently shaped spicules may be present at various tissue sites and in some instances even in the same tissue. Six different minerals are represented
among the pyurid spicules examined. They are vaterite, calcite, amorphous calcium carbonate, amorphous calcium phosphate, crystalline carbonate hydroxylapatite (dahllite) and amorphous fluorite. The spicules of Balhypera, Herdmania and Culeolus are monomineralic, whereas in Pyura
the mineralogy differs between as well as within species. In P. bradleyi three minerals occur in one type of spicule. The Pyuridae, like the aplousobranch Didemnidae and Cystodytes species, form only calcium minerals. The Pyuridae, however, do not form aragonite, which is the
only biomineralization product in didemnid and Cystodytes spicules. The stellate spicules found in species of Pyura and Didemnidae are morphologically similar but distinct in their mineralogy. This has to be taken into account when one attempts to trace the evolution of the Pyuridae
and Didemnidae by means of fossil spicules.
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