Abstract
On the basis of a histological investigation concerning 16 species of 4 holothurian orders, the homology of the so‐called problematic canal with the axial sinus on the one side and the dorso‐axial haemal strand with the axial gland on the other side is discussed. Combining these results with former investigations, the following conclusions are drawn. There is no axial complex as an organic unit represented within the class Holothuroidea. Nevertheless, this axial complex, formerly thought to be missing, is locally disintegrated into two parts, which are isolated from each other within the dorsal mesentery. One part is the stone canal—protocoelampulla—the other one the axial sinus—axial gland. Within the holothurians there is a tendency towards reduction of the protocoelampulla and the axial sinus. This recently aberrant structure of the axial complex can be deduced from an echinoid “plan”.
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