Morphology of the circulatory system in Caobangia billeti Giard, 1893 (Annelida, Sabellidae).

Caobangia billeti is an aberrant shell-burrowing annelid with a phoronid-like body plan and U-shaped digestive tract, an anterior anus on the dorsal side near the head, and reduced metamerism. The segmental dissepiments in C. billeti are absent, but the circulatory system is complex and consists of numerous lacunae and vessels. We asked the question how this modified circulatory system compares to that of other Sabellida. To that end, we collected C. billeti from the type locality and for the first time provide a detailed morphological investigation of these worms using both electron (TEM) and light microscopy. Our data show that, despite the significant modification of the body plan, the circulatory system is generally similar to that of other Sabellida representatives. The central circulatory system includes dorsal and ventral vessels, circular segmental vessels, and the peri-intestinal sinus, while the peripheral system includes the vessels of the branchial crown, collar vessels and esophageal plexuses, and transseptal vessels. However, compared to other representatives of the order Sabellida this species possesses four paired lateroventral vessels instead of two, and extravasal rather than intravasal hematopoietic tissue. The abdomen of C. billeti has lost all internal metameric structures, except segmental vessels, the number of which coincides with the number of uncinial tori. The direction of blood flow was reconstructed using vital and morphological observations. There are no specialized propulsatory organs, but all vessels pulsate, except for the branchial vessels, which are passively filled with blood. In the circulatory system, the direction of blood flow can be reversed (from the head to the back of the body inside the dorsal vessel and vice versa ventrally), which has not previously been described for other Sabellida.