Microscopic anatomy of the axial complex and associated structures in the brittle star Ophiura robusta Ayres, 1854 (Echinodermata, Ophiuroidea)

Abstract

Several details of the anatomy of the axial complex in brittle stars remain unknown, and there are many mismatching descriptions of its structure in different sources. The comparison of the ophiuroid axial complex with other classes of echinoderms is important for understanding of the phylogenetic relations in Ophiuroidea. We describe the organization of the axial complex of Ophiura robusta, compare it with other Ophiuroidea and analyse how the specific structure of the brittle star axial complex could appear in evolution. The standard technique of dehydration of material in alcohols of increasing concentration was used, followed by embedding material in paraplast and dissection. In the main components, the axial complex of Ophiuroidea fits other Asterozoa. But there are some important differences. The stone canal connects with the ambulacral ring from the outside, not from the inside. The somatocoelomic perihaemal ring is closer to the mouth than the axocoelomic ring. The axial complex lies between the genital coelom and the digestive tract. The gastric haemal ring is located on the outer side of the axial complex. The “pericardial” part of the axial organ is shifted to the oral side, but all its anatomical connections are retained: with the genital haemal ring, with the haemocoel of the body wall, with the gastric haemal ring and the vessels of the axial part of the axial organ. All these features could be explained as a result of shifting of madreporic plate along interradius CD from the aboral side to the oral side.