Rhynchocoel vessel in Cephalotrichidae (Nemertea: Palaeonemertea)

The morphology of the rhynchocoel blood vessel was studied in 26 specimens representing 14 species of the family Cephalotrichidae. In eight specimens the vascular system could not be traced because of their small size, poor fixation, or contraction during fixation. In the other 18 specimens, a short mid-dorsal rhynchocoel vessel was found. In Cephalothrix adriatica, C. hongkongiensis, C. kefersteini, C. oestrymnica, C. orientalis, C. rufifrons, C. cf. fasciculus and C. cf. simula, the mid-dorsal rhynchocoel vessel penetrates the dorsal side of the proboscis insertion, and extends backward abutting the mid-dorsal rhynchocoel wall (Type A). In C. filiformis sensu Iwata (1954 Iwata, F. 1954. The fauna of Akkeshi Bay. XX. Nemertini in Hokkaido. J Fac Sci Hokkaido Univ Ser VI, Zool., 12: 1–39.  [Google Scholar]), the vessel is pendant in the rhynchocoel above the proboscis (Type B). In C. filiformis s.str., the vessel runs in the proboscis wall, after passing through the proboscis insertion (Type C). Morphology of the rhynchocoelic vessel might have taxonomic significance in cephalothrichids.