Fig. 3From: The internal morphology of primary spines of extant regular echinoids in the tropical western Atlantic: a SEM atlasTretocidaris bartletti (A. Agassiz), USNM E11897, SE micrographs of broken sections through primary spines. a Stub A8. b Stub A8 (different specimen from a). c, d Stub A9, both ends of broken spine. All scale bars represent 1 mmBack to article page