The true relations between CaCO₃ hard parts and living tissues in the algae are poorly understood. It has been known for many years that calcification in the Rhodophyta (red algae) occurs within the cell walls of the organism, and that calcification of the dasycladacean Chlorophyta (green algae) partly encloses the plant within an external calcareous encrustation. However, the nature of calcification in the very important codiacean algae (siphonaceous algae, Order Caulerpiales of modern algologists) has not been investigated previously. Because it is important to relate preserved calcareous structures to the cell morphology of the parent organism, the genus Halimeda was studied in detail to aid in the interpretation and classification of fossil codiacean materi l.

Previous work by Lowenstam showed that calcareous skeletal material of Halimeda is composed of acicular crystals of aragonite, but no mention has been made of the relations between the aragonite and the living cell. Living Halimeda was collected along the Florida Keys and preserved in formaldehyde, impregnated with Vestapol plastic, sectioned with an ultramicrotome, and studied under an electron microscope. The aragonite was found to consist of acicular to blocky crystals ranging in length from 0.10 to 1.0 micron. The crystals occur as distinctly separate individuals suspended in a slurry outside of the cell walls which fuse to form a rigid calcareous structure on the death of the alga. No CaCO₃ was seen within the cells or cell walls. Halimeda is an alga composed of a sing e coenocytic, tube-shaped cell which branches repeatedly until the tiny cell tips (utricles) form the outer surface where photosynthesis occurs. Calcification occurs outside the cell walls but inside the uticle layer, forming a calcareous mold of the living cell. Therefore, fossil codiacean algae like Halimeda would not contain preserved parts of the cells or cell walls, but instead would consist of calcareous molds of the cell material. Openings formed by the removal of cellular material usually are infilled with sparry calcite in fossil codiaceans, whereas the intercellular aragonite usually is composed of dense, fine-crystalline CaCO₃. Fossil Codiacea may be classified safely on the basis of their external cell and utricle morphology.

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