'Tailed' Cephalopods

The anatomy and development of the tails at the posterior part of the mantle were studied in several groups of Recent and extinct coleoid cephalopods; substantial differences in their formation were revealed. Males of the Recent loliginid squid Alloteuthis spp. form their tail by increased growth of the anterior part of the gladius with simultaneous growth of the posterior mantle. As a result, the gladius rolls longitudinally in the tail forming a pseudoconus. The attenuated tail in males of the squid Lycoteuthis springeri (Lycoteuthidae) is supported from inside by the special rod-like apical vacuolated cartilage. Adults of both sexes of recent Onykia robsoni and O. robusta form a carrot-shaped flexible chitinous rostrum supporting the attenuated tail. Adults of several Jurassic belemnites formed an elongated epirostrum posterior to their orthorostrum; the structures differed in growth and microstructure. Counts of growth microincrements within the orthorostrum and epirostrum were used to date their formation and estimate the age of belemnites. The development of the long rigid tail and the corresponding shift of the fin to the middle part of the mantle streamline the body and possibly facilitate the animal's movement in the water by gliding. The analogous tail formation in several independent groups points to its adaptive nature for the development of a more mobile adult phase in species of coleoid cephalopods.