Use of Birefringence to Characterize Aplacophora Sclerites

The calcium carbonate sclerites of Aplacophora provide major taxonomic characters at both species and higher taxa levels. They are formed of aragonite crystals by a single cell. In all species of Chaetodermomorpha (chaetoderms or Caudofoveata) they are solid structures, and are either solid or hollow in species of Neomeniomorpha (neomenioids or Solenogastres). Sculpturing may be present. Size varies from a few to hundreds of microns in length and 1 to >10 mu m in thickness. Shapes vary widely. In chaetoderms, sclerites are elongate and leaflike, spatulate, or rod-shaped; in neomenioids, they are leaflike and ovate, or elongate needles, or fishhook-like, trowel-shaped, or curved in various ways. The morphology of aplacophoran sclerites from different regions of the body are described by line drawings indicating size and shape, by scanning electron micrography for imaging fine details and as an aid in interpreting line drawings, and, for solid sclerites, by birefingent patterns produced under a light microscope equipped with polarizing lenses. Line drawings and SEM images are standard practices in invertebrate taxonomy, but the use of birefringence is not so usual. The method is described here for Aplacophora, and the resulting images are compared to SEMs using species of Prochaetodermatidae genera for examples.