Ezhova O.V., Malakhov V.V. 2010. Three-dimensional structure of the skeleton and buccal diverticulum of an acorn worm Saccoglossus mereschkowskii Wagner, 1885 (Hemichordata: Enteropneusta) // Invertebrate Zoology. Vol.6 (for 2009). No.2: 103–116 [in English].

Moscow State University, Biological Faculty, Department of Invertebrate Zoology; Leninskie Gory, 1, bldg. 12, Moscow, 119991 Russia. E-mail: olga_ejova@mail.ru  vmalakhov@inbox.ru

KEY WORDS: Saccoglossus mereschkowskii, proboscis skeleton, buccal diverticulum, morphology, taxonomy.

ABSTRACT: We describe and illustrate the morphology of unpaired proboscis skeleton and buccal diverticulum of a hemichordate Saccoglossus mereschkowskii for the first time and compare them to homologous structures in other species. The proboscis skeleton is a unique structure found only in enteropneust hemichordates. It is made of cartilaginous-like material and supports the buccal diverticulum. The proboscis skeleton is a Y-shaped organ, which consists of the unpaired anterior plate and two posterior horns. The anterior part of the plate bears the conical rostrum with the two symmetrical supporting saucers on each side. The ventral side bears a central fossa. A pair of lateral wings extends on each side of the proboscis skeleton. The ventral keel passes along the ventral midline. The horns are curved symmetrical outgrowths separating the flattened diverticulum base from the buccal tube. The buccal diverticulum consists of three main parts: the flattened base, the isthmus and the hood-shaped anterior region, supported by the skeletal rostrum. The isthmus consists of the axial part with a small spherical lumen. The base of diverticulum anterior part has three outpocketings: one ventral and two lateral. The skeleton growths fit the appropriate depressions of the buccal diverticulum surface. The morphology of the skeleton and the buccal diverticulum in enteropneusts varies from species to species and can serve as a potentially useful taxonomic character.

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