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In an earlier paper (Goodey, 1924), the writer has given an account of the morphology of the adults of Œsophagostomum dentatum, the development and structure of the free-living larvæ and the biology of the ensheathed infective stage. So far, however, the parasitic larval stages have not been described. In the present paper some of the gaps in our knowledge of the complete life-history of the worm represented by these stages are filled in, not as completely as could be desired owing to the absence from the material of specimens showing the 3rd and 4th ecdyses. The worms found, however, show that the 4th stage larva has a provisional buccal capsule essentially similar to that found in 4th stage larvæ of O. radiatum (Marotel, 1908) and O. columbianum (Veglia, 1924), and in this respect is in line with certain other 4th stage larvæ of the Strongyloidea such as Ancylosloma duodenale (Looss, 1897), and Triodontophorus tenuicollis (Ortlepp, 1925), in which provisional buccal capsules built on the same general plan have been observed.

1. During the free-living development the larvæ of Triodontophorus tenuicollis undergo two moults separating three larval stages, the last stage remaining ensheathed in the cuticule of the previous stage.2. The development takes about four days when the eggs are cultured in horse fæces and charcoal at 26° C.3. The infective or third stage larvæ are climbers, and also show a marked resistance to desiccation ; they can also withstand variations in temperature from – 8° C. to 60° C.4. They do not penetrate skin.5. Infection of the host is probably via the mouth during grazing.6. The fourth-stage larva is described. It is free in the lumen of the colon, possesses a well-marked larval buccal capsule and three stout and pointed œsophageal teeth. Sex differences are now present.7. The adult buccal capsule develops in the form of a vesicle round the base of the larval buccal capsule.8. The probable mode of its parasitic development is given.9. A fourth-stage larva, probably that of either T. brevicauda or T. serratus, is described.