Autotomy of the Visceral mass in the feather star Himerometra robustipinna (Crinoidea, Comatulida).

The microanatomy of the attachment sites of the visceral mass to the calyx before and after visceral mass autotomy in the feather star Himerometra robustipinna was investigated. At the aboral site, the visceral mass is linked to the calyx by septa of the aboral coelom and is attached to the tegmen at the peripheral site. The connective tissue of the septa and tegmen contains cells resembling typical juxtaligamental cells of echinoderms, nerve cells, and bundles of axons. Visceral mass autotomy in H. robustipinna can be provoked by mechanical action and occurs relatively rapidly. Immediately after the visceral mass is gripped with forceps, the proximal pinnules are lowered and form a dense cluster covering the calyx. If the visceral mass is held for 20-30 s, the proximal pinnules are raised. At this time, the visceral mass has separated from the calyx and can be easily removed. During autotomy, the aboral coelomic septa are broken under the aboral wall of the subintestinal coelom, and the tegmen is ruptured at the interradii along the periphery of the calyx and at the base of the arms. The juxtaligamental cells probably participate in the alteration of the connective tissue and the breakage of the septa and tegmen. The granules of juxtaligamental cells swell, develop an electron-transparent halo, and are released into the extracellular matrix. In general, our results suggest that separation of the visceral mass in H. robustipinna is characterized by all the features of autotomy.