SYNCYTIAL ORGANIZATION OF ACANTHORS OF POLYMORPHUS-MINUTUS (PALAEACANTHOCEPHALA), NEOECHINORHYNCHUS-RUTILI (EOACANTHOCEPHALA), AND MONILIFORMIS-MONILIFORMIS (ARCHIACANTHOCEPHALA) (ACANTHOCEPHALA)

The fine structures of immature and of developed shelled acanthors of three species belonging to the three subgroups of the Acanthocephala w ere investigated. Acanthors are surrounded by four eggshells (embryoni c envelopes) and are composed of three syncytia: a frontal syncytium, a central syncytium, and an epidermal syncytium. Neither a sense organ nor a nervous system has been found. The central syncytium shows a ma ss of condensed nuclei and 12 decondensed nuclei and gives rise to 10 anterior/posterior subepidermal myofibrillar systems and 2 oblique ret ractor muscles. Circular muscles are missing. A single decondensed nuc leus can be assigned to each of the 12 muscular systems. The epidermal syncytium embeds the other two syncytia and forms the wrinkled epider mis, which shows an extracellular glycocalyx and intrasyncytial conden sations. Prominent recurved hooks, which mark the anterior end of each acanthor, and body spines are intraepidermal differentiations. Partly branched tubular infoldings of the epidermal plasma membrane of the a canthor exist and represent precursors of the pore ducts typical of th e adult epidermis. Autapomorphies in the ground pattern of the monophy lum Acanthocephala are the four eggshells, the early development of th ree syncytia, the condensed nuclei in the central syncytium, and the d ifferentiation of ten longitudinal muscle bands and two muscle retract ors and of intraepidermal hooks and spines. The syncytial organization of the epidermis with intraepidermal skeletal condensations and infol dings of the apical plasma membrane are characteristics inherited from a stem species common to Acanthocephala, Seison, and Rotifera.