ИСТИНА

The fine structure of the jaw apparatus and the ventral pharyngeal organ in Histriobdella homari - a tiny ectocommensal known from lobsters - has been studied by using routine morphological tools (SEM, TEM, CLSM). The Histriobdella jaw ultrastructure is highly comparable to those of Dorvilleidae and juvenile jaws of Onuphidae, and represents a sclerotized cuticle. In the process of development of the maxillary apparatus, mandibles, maxillae II and unpaired dorsal carriers are formed first, and later the remaining maxillae and transverse carriers appear. Notably, the general structure of the histriobdellid maxillary apparatus resembles the conditions known for Oenonidae,. The pharyngeal muscular apparatus differs - not only due to the very small size of Histriobdella, but also because the histriobdellid maxillary protraction occurs due to straightening of the dorsal carriers (dorsal rod) and thus requires a differing muscular scaffold. Based on our investigations, the general muscular apparatus of the jaws and ventral pharyngeal organ is unique. At the same time, our ultrastructural data (TEM) show that the main components of the Histriobdella muscular system fit into the general scheme of the ventral pharyngeal organ in annelids. Nevertheless, our data provide new insights helping to understand the evolution of eunicid jaws in general.