Echiurida is a small group of marine benthic invertebrates that burrow in sediments and live a hidden lifestyle. Investigation of the morphology and anatomical features of various organ systems allows better understanding of the biology of these enigmatic animals, many of which are deep-sea species. The morphology and microscopic anatomy of the deep-sea echiurid Protobonellia zenkevitchi Murina, 1976 have been studied using the light microscopy and histology methods. The body of P. zenkevitchi is divided into a proboscis and trunk. The ciliary grooves and large vacuolated cells in the connective tissue of the distal part of the proboscis suggest a specific position of the proboscis on the sediment surface and a mechanism for sorting food particles. It has been shown for the first time that the coelom is not subdivided into compartments. In the digestive tract, an unusual part of the midgut has been found that was previously unknown in echiurids; it probably performs the function of food storage. This part contributes to thorough food digestion, which is important in the oligotrophic conditions of greater depths. The circulatory system of P. zenkevitchi lacks the neuro-intestinal and ring blood vessels. The oocyte storage chamber of the gonoduct has a pore, which apparently connects its cavity with the trunk coelom, but lacks a specialized and well-expressed part, the androecium. A comparative analysis of the microscopic anatomy shows that many organ systems (muscular, coelomic, circulatory, excretory, and reproductive) in P. zenkevitchi have a simpler organization compared to those in other echiurids. This can probably be explained by the small body size of P. zenkevitchi and the great depths of its habitat. At the same time, P. zenkevitchi possesses some unique anatomical characteristics related to the features of the biology of this deep-sea species.
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