Fig. 1From: The internal morphology of primary spines of extant regular echinoids in the tropical western Atlantic: a SEM atlasEucidaris tribuloides (Lamarck), USNM 10734, SE micrographs of broken sections through primary spines. a Stub A1, b Stub A3, c Stub A2, d Stub A1 (this is a different specimen from a, but on the same stub). All scale bars represent 1 mmBack to article page