Understanding ecdysis in Early Cambrian Scalidophoran Worms.
[sciencythoughts.blogspot.com]
The epidermis is a fundamental cellular tissue of Metazoans (Animals) which acts as an interface between the external medium and the body interior. It includes epithelial cells and extracellular material. Epithelial cells form a tissue with complex junction systems and secrete two types of extracellular substrates: basally, the extracellular matrix, and apically, the glycocalyx (layers of proteins and sugar) and the cuticle. Although absent in Placozoans and Ctenophores, the glycocalyx plays a crucial role as a matrix on which other extracellular molecules (e.g. chitin, collagen) can anchor and eventually form a cuticular structure. A cuticle occurs in all present-day Ecdysozoans (Nematoida, Scalidophora, Panarthropoda), with a consistent three-layered structure. Ecdysozoans display a great variety of cuticular structures often associated with sensory organs (e.g. setae). Cuticular moulting is common to all extant representatives of Ecdysozoa and is a key diagnostic feature of the group. Exuviae found in early Cambrian Scalidophoran worms from China (roughly 535 million years old) suggest that ecdysis has a very ancient origin and evolves through a suite of genetic innovations in the three Ecdysozoans clades (Scalidophora, Nematoida, Panarthropoda).
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