Acanthocephala.—The early encysted stages of several species of Thorn-headed worms (Acanthocephala), belonging to the family Echinorhynchidæ, have been reported from snakes belonging to very different genera, such as Boa, Tropidonotus, Zamenis, Drymobius, Xenodon, Dipsadomorphus, Oxyrhopus, Erythrolamprus, Diemenia, Naja, Elaps, Vipera, Lachesis. Their further development probably occurs in ophiophagous birds. Thus, Echinorhynchus oligacanthoides, Rud., the immature stages of which occur encapsuled within the body cavity of Lachesis lanceolatus and other neotropical snakes, when adult is found attached to the intestinal mucosa of Milvus bidentatus.
Nematoda.—The roundworms (Nematoda) so far described from snakes belong to the families Ascaridæ, Strongylidæ, Trichotrachelidæ, and Filariidæ. Some of the genera belonging to these families, such as Cucullanus, Nematoxys, Oxysoma, are as yet represented by a single species in a single host; others, such as Ascaris, Polydelphis, Heterakis, Strongylus, Diaphanocephalus, Physaloptera, Trichosoma, number already several species more or less widely distributed.
Eelworm infection (ascariasis) is very common in snakes, and not infrequently the infection is a heavy one; Sambon twice found over fifty specimens of Polydelphis in Puff-adders (Bitis arietans). This investigator has shown that the snake eelworms undergo an encysted stage of development within the body cavity of their hosts before migrating into the intestinal lumen for the purpose of fertilization and oviposition. Thus, Redi was quite right in considering the immature, encysted forms found in one of the livers of his double-headed Asp as belonging to the same species of eelworm (Ascaris cephaloptera) as that which the snake harboured in its intestine.
Professor A. Railliet, whilst examining specimens of Polydelphis which had been preserved for nearly two months in a 3 per cent. solution of formalin, found that the ova within their uterine tubes had undergone development, and still contained living embryos; indeed, some of these hatched under the microscope, and moved very actively in the preserving fluid. This is in no way surprising, because even after several years of preservation in formalin solution the embryos of other species of eelworms (Ascaris equorum, A. marginata) have been found in a living condition.
Trematoda.—The Flukes (Trematoda) of snakes, so far described, belong to the following genera: Agamodistomum, Astiotrema, Brachylaimus, Cotylotretus, Dicrocœlium, Diplodiscus, Distoma, Halipegus, Lecithodendrium, Metorchis, Opisthogonimus, Opisthorchis, Plagiorchis, Saphedera, Telorchis, Tetracotyle, Zeugorchis.
Cestoda.—Save a few larval forms (Cysticercoides, Piestocystis, Sparganum), the known tapeworms (Cestoda) of the Ophidia belong to the genera Bothridium and Proteocephalus.
Protozoa.—Numerous species of Hæmogregarines have been described from snakes. As a rule the forms seen in the peripheral blood are sporonts, the schizogonic cycle occurring in the lungs. The sporonts do not greatly alter their host cells; they are invariably doubled up within a more or less thick capsule. Some species show a marked sexual differentiation, others not. Trypanosomes, Spiroechaudinniæ, and Plasmodidæ have also been described from the blood of various snakes.
Within the alimentary tube have been found species of Trichomonas and Caryospora.
Bacteria.—Acid-fast bacilli have been described in tubercular lesions found in snakes by Sibley, Gibbs and Shurley, Shattock, Hausemann, and Sambon.
The so-called “canker,” which so frequently develops in the oral cavity of captive snakes, is also a bacterial disease, due to a specific bacterium of thick, rod-shaped form.