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The ophiuroid arm contains a series of vertebral ossicles that form an articulated internal skeleton. Ontogenetic, serial, and interspecific variation in these skeletal elements are investigated using morphometric data from 35 species of brittle-stars (Order Ophiurae). Multiple ossicles were sampled from each individual and several individuals were sampled from each species to reconstruct serial and ontogenetic changes in vertebral morphology. Within species, ontogenetic and serial allometries are not statistically different. These data support ‘Jackson's law of localized stages’ (Jackson, 1899; Clark, 1914), which proposes that serial variation along the arm reflects ontogenetic stages of ossicle growth.

A multivariate analysis of interspecific variation shows two major vertebral forms: ossicles with a proximal depression and distal keel, and ossicles lacking these features. Variation within these groups is largely continuous, but individual species show distinct shape differences and unique allometric patterns of serial variation. These results suggest that vertebral ossicle variation among species can be described by: 1) variation in initial shape; and 2) variation in the allometric trajectory along the proximal-distal axis.

In all species, the most proximal ossicles within the disk show a non-keeled morphology. In species with keeled arm ossicles, however, there is an abrupt transition within the disk between non-keeled and keeled vertebral forms. A single ossicle, having features of both vertebral types, occurs at this site. The taxonomic distribution of the two vertebral forms and the anatomical transition between forms is discussed with reference to current classification systems and recent phylogenetic schemes for the Ophiuroidea.