Comparative morphology and cytology of siphons and siphonal sensory organsin selected bivalve molluscs

The form, anatomy and cytology of the waterpassages in siphon-possessing an d siphon-lacking species of a selected group of bivalve molluscs from the R ed Sea (Callista florida, Circe currogata, C. crocea, Lucinia dentifera, Do sinia histrio, Pitar hebraea, Tridacna maxima, Pteria aegyptica), and from the Mediterranean Sea (Spondylus spinosus, Pinctada radiata, Pinna nobilis, Donax trunculus, D. semistriatus, Mactra stultorum, Tapes decussatus, Pert icola lithophaga, Brachidontes pharaonis) were studied by means of light an d electron microscopy. In the mytiloids, ostreoids and ptrioids studied, th e water-passages are covered by a ciliated epithelium and the few tentacles around their openings are identical to those found on the mantle edges. Co ntrary to this, in the veneroids studied, the siphons are covered by a micr ovillar epithelium and their tentacles, especially of the inhalant siphon, reveal characteristic branching. The so-called Type I, Type II and Type III ciliated sensory organs on and within the siphons are similar to those obs erved in other bivalves, differing from each other in the dimension, number and form of cilia. Ill the studied mytiloid species only Type I and Type I I organs were found. The form and structure of the siphons and the distribu tion of sensors on them call be used as markers for ecological as well as p hylogenetic considerations.