Like all other animals, snakes are infested with a multitude of vegetable and animal parasites, both external and internal. About 300 species of Ophidian parasites have been recorded; yet our knowledge of them is very imperfect. Although some 2,000 species of snakes are known, parasites have not been recorded for more than 168 species, and in the great majority of these (102) only a single parasite: a tick, a hæmogregarine, or some intestinal worm. Owing to the more frequent opportunity of dissecting them, the common menagerie snakes have yielded better records, notwithstanding the fact that they usually lose most of their parasites through constant handling, prolonged fasting, and artificial surroundings. Thus, we have a list of thirteen species for the Indian _Python molurus_, and one of twenty-two species for the _Boa constrictor_. But no systematic search appears to have been attempted, save, perhaps, in the case of a few European species.
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_.