Variation in morphological characters seen in a large collection of specimens of the genus Echinosigra from the Rockall Trough can be arranged in a continuous ontological series. This growth series embodied forms agreeing with Mortensen's (1907) descriptions of both E. phiale and E. paradoxa. Bivariate biometrical study showed a well-defined allometric relationship of test length to other body dimensions, particularly the width of the ‘head’ and the width and height of the ‘body’. Immature specimens corresponded to E. phiale whilst the typically elongated ‘neck’ of adults corresponding to E. paradoxa was developed, along with the opening of the genital pores, at around a test length of 20–24 mm.
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