THREE TEXT-FIGURES ARiD TWO PLATES
This gregarious rhabdocoele was found in the ponds in the vicinity of the University of Virginia. These animals occurred on plants and other submerged objects, especially at the outlets of the ponds where there was a quantity of moss which was kept damp by the falling of the water. These animals are creatures of a crawling habit and never seem t,o leave the surface of some object. I n the laboratory they crawl along the bottom or sides of the aquaria or some submerged surface and never leave this unless firced to do so by some mechanical means. Later in this paper we will point out what we think to be a correlation between the disposition of the organs of special sense in a creeping form like this as compared with the disposition of such organs in a free swimming form such as Microstoma caudatum Leidy.
Specimens used for this histological study have been fixed in aceto sublimate, at various temperatures, Bouin's fluid, Carnoy's fluid, chrom-formalin mixture, and chrom-aceto-formalin mixture. Without exception, we find that the last named fixative is the best for this purpose. Specimens were fixed in this chromaceto-formalin mixture2 from twenty to thirty minutes, washed in several changes of tap water, and then carried through to paraffin. We find that it is best to handle the specimens separately and not to work on a large number at one time. Sections, 3 to 10 microns thick, were stained in iron haematoxylin with 1 Thc authors ale mutually rcsponsiblc for the observations and dediictions 2 16 parts 1 per cent chromic acid, 8 parts 10 per cent formalin, 1 part glacial recorded in this p a p c ~ arctic. This mixtnre has t o be used as soon as it is mixed.