(1) Staining methods using paraldehyde-fuchsin, chrome-haematoxylin, and alcian blue or astra blue following oxidation, have demonstrated the shape of the hook and other structures on the cephalic space of microfilariae. These structures are described for three species of the genus Brugia, B. malayi, B. pahangi and B. patei, and are compared with the related structures found in the genera Wuchereria, Loa, Cardiofilaria, Onchocerca, Mansonella, Dipetalonema and Litomosoides.
(2) In Brugia the hook of the microfilaria is V-shaped and is supported basally by a pair of transverse basal pieces which are closely associated with an oral ring. Similar structures are found in the microfilariae of Wuchereria and Loa except that in Loa the hook is supported below by a semi-circular arch. In Brugia there is a row of three spines transversely on the cephalic space opposite the hook complex, the spines being markedly shorter in B. patei than in the other two species; in Wuchereria there is a single transverse row of four spines, and in Loa there are two or three transverse rows of spines, No spines arranged opposite the hook complex were observed in the other genera. The hook and associated structures in Cardiofilaria are similar in arrangement to those of Loa, and the small structures in Mansonella can possibly be derived from a similar arrangement. In Onchocerca the hook has the appearance of a broad plate and in Dipetalonema perstans part of the surface of the cephalic space appears to be involved with the small pointed “hook”. In Litomosoides the tip of the cephalic space is covered by a helmet-like structure carrying a pointed beak-like projection.
(3) It is suggested, from their staining reactions, that the cephalic structures of microfilariae are particularly rich in disulphide bonds, suggesting the presence of cystine.
(4) The staining methods also demonstrate an internal thread of tissue in microfilariae which extends in Brugia from the tip of the cephalic space back to the innen korper. A similar thread is present in the microfilaria of Wuchereria, Loa, Cardiofilaria and Mansonella but was not observed in the other genera, although a short thread of tissue was stained in the cephalic space in Onchocerca and Litomosoides. In Brugia, the buccal capsule, the pharynx and the pharyngo-intestinal valve develop around the pharyngeal thread of the microfilaria. The extreme end of the thread, around which the pharyngo-intestinal valve develops, differs in its staining reactions from the rest of the thread.
(5) The staining reactions of the pharyngeal thread in the microfilaria of Brugia show resemblances to those of “elastic” fibres in other invertebrates and oxytalan fibres of vertebrates. The pharyngeal thread appears to contain an acid mucopolysaccharide component.
(6) The possible functions of the cephalic and pharyngeal structures in the microfilariae are discussed briefly.
(7) A staining routine for demonstrating the structures described in this paper is given in an appendix.