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Show 480 MR. F. E. BEDDARD ON THE ATRIUM AND [May 16, ever, there are stronger arguments than those to be derived from considerations of this kind. I have pointed out that in Megascolex newcombei the atria have the compact appearance of the corresponding organs of Acanthodrilus, to which also they show some slight approximation in outward form ; this is further shown in the slight branching of the lumen of the gland. A better instance still has lately come under m y observation. In the remarkable genus Biplocardial of Gar man (11) the atria appear on a dissection to resemble those of other Acanthodrilidae ; they are long tubular structures terminating in a short muscular duct. They have, however, a curious spotty appearance, small white rounded masses being imbedded in a darker ground-substance. They are also not quite so regularly tubular as they are in all other Acanthodrilidae which I have dissected ; towards the junction with the muscular duct the tubes are wider, and everywhere the margin is slightly crenate. In transverse sections the reason for these divergences from the normal Acanthodrilid structure was apparent: the white spots are aggregations of the pear-shaped glandular cells, which no longer form a regular investment of the lining epithelium of the tube; and here and there the lumen has inconspicuous diverticula into which the pear-shaped cells pour their secretion. The whole gland in fact shows a commencement of that branching which is so characteristic a feature of the atrium in the Perichaetidae. A little more development of the tendency to the aggregation of the pear-shaped cells and we should have an atrium exactly like that of the Perichaetidae. If it be reasonable on a priori grounds alone to regard the glandular appendices of the sperm-ducts in the Megascolicidas as homologous throughout, it is at least equally reasonable to seek for detailed resemblances in the atria in the different genera which make up the family of the Tubificidae. At first sight there seem to be considerable differences. In Tubifex itself, and Limnodrilus and some other genera identical with Tubifex in these particulars, the atrium is long and has the well-known " Cement-Driise " attached to it; this gland is limited in extent, though varying somewhat in different genera, and is superadded to the lining epithelium of the atrium ; Vejdovsky (3) has shown that it is developed from the lining epithelium. In Telmatodrilus (12) there is a series of these outgrowths of the atrial epithelium, which deck the entire atrium, being symmetrically arranged. Finally, in m y genus Branchiura (7) the atrium is closely invested by a thick layer of pear-shaped cells, which are separated from the lining epithelium by a muscular coat of circular fibres. Ilyodrilus appears to possess an atrium of the same kind but minus the muscular fibres. In Tubifex the muscular coat exists but it is less developed than in Branchiura; it is, moreover, deficient at the point where the "Cement-Driise " is grafted on to the atrium. The only remaining genus of Tubificid which shows any differences from these 1 I am indebted to tbe kindness of Dr. Benham for the opportunity of examining this worm. |
