1. The terms cilia, rod-border, brush-border, and other terms used in this paper are defined.
2. The epithelium lining the mid-gut of Melinna is described with special reference to the cell-border. Variation between different regions of the gut are noted. There are two main types of epithelia: (i) ciliated, (ii) rod-bordered. The presence of a peritrophic membrane in the middle region of the stomach is noted.
3. A description of the mid-gutepithelium of Locusta and Chironomus is given, and the essential similarity between the brush-borders of the two is stressed.
4. The ciliated effect described by Vignon in Chironomus is shown to be due to the presence of motile bacteria. 5. Prom a thorough survey of the literature and from our own observations, it is concluded that there are two main types of cell-borders: A. Motile, i.e. ciliated; with which basal corpuscles, derived from division of the centrosomes, are always associated.
B. Non-motile.--It is held that these are of two distinct types: (i) Rod-border, where the elements are probably homologous with the basal segments of cilia. This type occurs in Melinna, Lumbricus, and probably in other animals, and it is possible that the striated free border found in the epithelium of the vertebrate intestine is of a somewhat similar nature.
(ii) Brush-border, where the elements are simple filaments and bear no relation to cilia. This type is found typically in insects and in a slightly modified condition in the uriniferous tubules of vertebrates.
Both motile and non-motile elements lie above the free surface of the cell which is marked by the limiting membrane.
6. Mention is made of the brush-border epithelium of the Malpighian tubiiles of insects.