The microscopic anatomy and ultrastructure of the contractile vessel in the sipunculan Themiste hexadactyla (Satô, 1930) (Sipuncula: Sipunculidea)

The microscopic anatomy and ultrastructure of the contractile vessel of the sipunculan Themiste hexadactyla (Satô, 1930) from Vostok Bay (the Sea of Japan) were studied by histological and electron microscope methods. The ultrastructural features of the internal (endothelium) and external (coelothelium) lining of the contractile vessel are described and illustrated. Numerous macromolecular filters, the so-called “double diaphragms,” were found in the external coelothelium facing the cavity of the trunk coelom. This suggests a possible filtration from the tentacular coelom into the trunk coelom though the contractile vessel wall. The microscopic peculiarities of the main tube of the contractile vessel and its numerous lateral branches twining around several internal organs are described in detail. The contractile vessel is polyfunctional: it can act as the main reservoir for the cavity fluid during the withdrawal of the tentacular crown and performs the functions of the distribution system in sipunculans.