Urticating Organs of Planarian Worms

Abstract

THERE exist, as is well known to all comparative anatomists, in the skin of most planarian worms certain rod-like bodies (Stäbehenkörperchen of German authors) concerning the function and homologies of which there has been considerable speculation and difference of opinion. By some authors these bodies, which always at an early stage of their existence are contained in cells, “the rod-cells” have been compared to the thread-cells or nematocysts of cœlenterata, the rod-cells being considered homologous to or possibly homogenous with these cœlenterate nematocysts. In the July number of the Quarterly Journal of Microscopical Science, vol. lxvii., new series, 1877, I published a paper on the structure of several forms of land planarians obtained by me during the voyage of H.M.S. Challenger. In this paper is described and figured the structure of the rod-cells of several genera of land planarians as observed in the fresh and living condition. In an American form Geoplana flava and also in a Geoplana from New Zealand and a Rhynchodemus from the Cape of Good Hope rod-cells were observed in which the rods are much longer than the cells and are in their quiescent condition coiled spirally within the cells (l.c., Pl. xx. Figs. 15, 20, 21, 22, 23), but which rods are shot out from the cells and protruded for a long distance beyond the surface of the epidermis when the animal is compressed or irritated. Such probably is the mature condition of the cells in question in all land planarians. Mecznikow has described a somewhat similar form of cell as existing in his Geodesmus bilincatus.