Arm structures of the ophiomyxid brittlestars(Echinodermata: Ophiuriodea: Ophiomyxidae)

The arms of Ophiomyxidae are ensheathed in two strata of connective tissue that
obscure the underlying skeleton. The arm integument of Ophiomyxa flaccida, 0. tumida and
Ophiobyrsa serpens is composed of a thin layer of feltwork connective tissue and a thick
layer of dense fibrillar connective tissue. The arm of Ophiogeron supinus is largely feltwork
connective tissue fibers with a predominance of interfibrillar matrix. Comparative
study of the arms of these four ophiomyxid species reveals that there is an inverse relationship
between the thickness of the connective tissue and the size of skeletal elements.
In Ophiomyxidae, connective tissue plays a supportive role that in other brittlestars is
fulfilled by calcite. Behavioral observations indicate that the connective tissue exhibits
variable tensility, a feature that may contribute to the skeletal properties of the integument.
The functional and systematic significance of the arm structure of ophiomyxid
brittlestars is discussed, and analogous specializations of Paleozoic ophiuroids are reinterpreted
based on ophiomyxid adaptations.