The singular monkey known as _Tarsius spectrum_ is liable to be attacked by a filaria (_F. lævis_), which is found beneath the skin. At least two species of Echinorhynchus (_E. spirula_ and _E. elegans_) are known to infest monkeys. I have carefully examined and figured _E. elegans_ in the ‘Zoological Society’s Proceedings,’ from specimens supplied to me by Dr Murie. They had been obtained from a pinche monkey (_Hapale œdipus_) from New Granada. I am under the impression that Diesing’s original description of this parasite is the only one that exists. I have gone over his numerous memoirs contributed to the Vienna Academy, but can find nothing beyond the specific characters given in his ‘Systema.’ All the specimens in the Vienna Museum, whence his description is taken, were collected by Natterer. They were procured from the marakina (_Midas rosalia_), from two other true marmosets (_Hapale ursula_ and _H. chrysoleuca_), and from a squirrel monkey or tee-tee (_Callithrix sciureus_). In the monkey which died at the Zoological Society’s Gardens the cause of death was not clearly due to the parasites; nevertheless, the mucous layer of the intestine, to which the entozoa were attached, showed deep conical pits or depressions at the spots where the worms had anchored themselves. During the perfect retraction of the proboscis of this Echinorhynchus the centre is represented by a wide opening which communicates with a cavity beneath. The end of the neck thus forms a sort of collar, or rosette, made up of rays arranged like the spokes of a wheel. When the proboscis is exserted this collar is more or less convex, but it becomes slightly concave when the proboscis is retracted. Diesing recognised twenty-four rays; they probably vary from that number up to twenty-eight, at least I counted twenty-seven in my specimens. During exsertion the proboscis forms a nipple-like projection. According to Diesing it supports three rows of hooks, but I certainly saw four rows. When separately magnified these hooks present very different appearances as to size and contour. These variations I believe to be due to growth. The largest hooks measured about 1/200″ in length.