The Median Tentacle of the Larva of Lingula Anatina (Brachiopoda) From Queensland, Australia

Abstract

The median tentacle of the larvae of Lingula anatina has been investigated in order to obtain information about the morphological implications of its function and its relation to the paired tentacles of the larval lophophore. The median tentacle is a larval sensory organ with apical, intraepidermal collar receptors and a basiepidermal nervous system (apical ganglion). The monociliated muscle cells in the median tentacle form a compact musculature; where cilia emanate from the single cells, small compartments surrounded by three epithelial muscle cells can be observed. The number of cavities corresponds with the number of muscle cells. In the proximal part of the median tentacle these muscle cells build a continuous transition to the muscle cells in the paired tentacles; an extracellular matrix could not be found in the contact-region between both muscular systems. Because of this continuity the muscle cells in the median tentacle must be regarded as part of the mesosome. The unpaired median tentacle is restricted to the larvae of all species of the Lingulida and Discinida within the inarticulate brachiopods and is lacking in all other brachiopod larvae. Because it is also unknown in Phoronida, it could be regarded as a synapomorphic characteristic of the supraspecific taxa Lingulida and Discinida.