Skeletal variation related to arm regeneration in Metacrinus and Saracrinus, Recent stalked crinoids

Variations in the number of skeletal elements in the proximal portion of the arms (primibrachials) of Metacrinus rotundus Carpenter and Saracrinus nobilis (Carpenter) are compared. Older specimens of Metacrinus rotundus show increased variation in number of elements along with a tendency toward reduction, whereas the number of elements remains nearly constant in Saracrinus nobilis. The variation in Metacrinus rotundus is attributed to inaccurate arm regeneration following autotomy. Separation of Saracrinus from Metacrinus on the basis of number of skeletal elements is therefore possible, but caution must be used in the application of taxonomic characters which might be influenced by regeneration.