Centrids, a pair of asymmetrically arranged sense organs in Ascaris suum (Nematoda, Ascaridoidea)

Two prominent, asymmetrically placed cuticular somatic sensilla, called cen trids, are reported in Ascaris suum Goeze, 1782, the pig roundworm. The rig ht centrid is situated much more anteriorly on the body than is the left on e. The centrids are globular in the fourth-stage larva and obviously void o f an apical pore, suggesting at least a tactile function. In adult worms, t he centrids are platelike, lacking a globular expansion. The observation on the presence of asymmetrically placed centrids in A. suum gives further im petus to the importance assigned to sense organs in the classification and identification of nematodes. The name centrid was originally chosen to indi cate the placement of the papillae in the midbody region of worms. The name centrid, rather than, e.g. postdeirid, is proposed to be used when denotin g asymmetrically oriented midbody sensilla among the Ascaridida and papilla e, when shown homologous to these, of species within the Rhabditea generall y. This proposal is in line with the name "Mittelkorperpapillen" originally adopted to denote homologous sensillae in Cucullanidae (Seuratoidea) by To rnquist in 1931.