It is interesting to notice that it was the finding of an Ophidian parasite which prompted Francesco Redi to write his famous "Observations on the Living Animals which are found within Living Animals." This work, a veritable treatise of comparative parasitology, published in 1684, caused the great naturalist, physician, and poet to be regarded as the father of that science. He tells us that in dissecting a curious dicephalous _Vipera aspis_, caught at Pisa, he found within the intestines a number of roundworms (_Ascaris cephaloptera_), and on the surface of one of the two lobes of the liver five cysts enclosing a small worm, which he rightly ascribed to the same species. The parasites of snakes are here enumerated by Dr. L. W. Sambon, in systematic order. ARTHROPODA.--Two families of the class Arachnida, the Ixodidæ and the Linguatulidæ, furnish numerous species parasitic on snakes. Of the Ticks (Ixodidæ) we find, as a rule, species of the genera _Amblyomma_ and _Aponomma_, the latter genus being almost entirely confined to Reptiles. A single species of the genus _Hæmaphysalis_ (_H. punctata_, Can. and Franz, 1877) has been reported once from _Vipera aspis_. A few larval forms found on various snakes have been reported under the generic name _Ixodes_, but they probably belong either to _Amblyomma_ or _Aponomma_. The Ophidian Tick-parasites, like those of mammals, birds, lizards, and tortoises, appear to be in many cases the means of transmission of protozoal infections from snake to snake. The Tongue-worms (Linguatulidæ) are, without doubt, of the greatest possible interest. Their systematic position has ever been a puzzle to zoologists, and even now is a matter of controversy. They have been looked upon as Hirudinea by Winsberg (1765), Cestoda by Chabert (1787), Acanthocephala by Humboldt (1808), Trematoda by Rudolphi (1809), and Nematoda by Nordmann (1832). It was Van Beneden (1848) who first recognized their Arthropod nature, but he placed them amongst the Crustacea. Schubärt (1853) suggested that their proper position is amongst the Mites (Acarina), and Leuckart (1860) adduced important anatomical and embryological evidence in support of this view, which was confirmed by Railliet in 1883 and by Sambon in 1910. No less than three out of the four genera of Linguatulids so far established are represented by species parasitic on snakes. They are the genera _Porocephalus_, _Reighardia_, and _Raillietiella_. The genus _Porocephalus_ is of special interest, because some of its species, such as _Porocephalus armillatus_, a parasite of African Pythons (_Python regius_, _P. sebæ_) and Puff-adders (_Bitis arietans_, _B. nasicornis_, _B. gabonica_), and _Porocephalus moniliformis_, a parasite of Oriental Pythons (_Python molurus_, _P. reticulatus_), are, in their nymphal stage, deadly parasites of mammals, including man.