GASTROTRICHA, a small group of fairly uniform ani mals which live at the bottom of ponds and marshes, hiding amongst the recesses of fresh-water plants and eating organic debris and Infusoria. They vary in size from one-sixtieth to one three-hundredth of an inch, and move by long cilia. Two ventral bands of regular transverse rows of cilia are usually found. The head bears some especially large cilia. The cuticle which covers the body is here and there raised into overlapping scales which may be prolonged into bristles. The body, otherwise circular in section, is slightly flattened ventrally. There is a protrusible pharynx armed with eversible re curved teeth. This leads to a muscular oesophagus with a tri radiate lumen, which acts as a sucking pump and ends in a fun nel-valve projecting into the oval stomach. The nitrogenous excre tory apparatus consists of a coiled tube on each side of the stomach, ending internally in large flame-cells. A cerebral gang lion rests on the oesophagus ; it is continued some way back as two dorsal nerve trunks. In some species there are eyes. The two ovaries lie at the level of the juncture of the stomach and rectum. The large eggs are laid amongst water weeds. The male reproductive system is little known. The Gastrotricha are her maphrodite. The group is divided into two sub-orders, the Euich thydina, with a forked tail, and the Apodina, in which the tail is not forked. About 5o species are known. The group shows no clear affinities with any of the great phyla.