The question as to whether Ascaris from the pig may be transmitted to man, and its converse, have long been recognised as being of considerable public health importance. Hitherto the evidence, both epidemiological and experimental, is somewhat in favour of the belief that human Ascaris and Ascaris from the pig are not interchangeable as to their respective hosts. Experimental evidence in support of this was supplied in 1922 by Koino, who swallowed a large number of embryonated eggs from the pig form, and although severe pneumonic symptoms were set up by the migrating larvæ, the latter did not complete their development, and no intestinal infestation followed. A similar negative result was obtained by Payne, Ackert and Hartman (1925), from infection experiments performed on themselves and on a rhesus monkey; they also fed large numbers of human Ascaris eggs to pigs but without success. Some epidemiological evidence has also been furnished by these workers, for they indicated that although in Trinidad and Arouca the circumstances were favourable for reciprocal infection, yet the incidence of Ascaris in man and pigs in these regions in no way suggested that such a condition of affairs obtained.

It has been, for some time, a recognised fact that intense eelworm infections are, in some instances, accompanied by only slight damage to the plants, while in other areas where the infection is much less intense, the plants show every symptom of “potato sickness” and the crop is a complete failure.

While it is true that various lists of rat parasites have been published these, however, deal only with individual Classes of helminths. Notably amongst such publications are those by Hall (1916) dealing with the Class Nematoda, by Meggitt and Subramanian (1927) who treat of the Class Cestoda and by Dollfus (1925) who reviews the species of the Class Trematoda. It was suggested to me by the Imperial Bureau of Agricultural Parasitology that the collection of notes, records and information concerning the helminth parasites of common rats into one single work would be useful and the information contained in the three compilations mentioned above has been largely drawn on in forming the nucleus of this paper. Other publications, found helpful in providing additional information, are those by Balfour (1922) who deals with the parasites of wild rats in England, Baylis (1922), Cleland (1918), Fielding (1928), Johnston (1918), who summarises the knowledge concerning the parasitism of rodents in Australia, Meggitt (1924), Moll (1917), Shipley (1908), and Yorke and Maplestone (1926). For the more recent records, not included in any of the foregoing publications, the Vermes section of The Zoological Record and the “Répertoire des Genres nouveaux et des Espèces nouvelles,” appearing in each number of the Annales de Parasitologie humaine et comparée were consulted advantageously. The checking of many records, at least of the earlier ones, was accomplished by means of Stiles and Hassall's invaluable Index-Catalogues (1908, 1912, 1920).

1. A preliminary account is given of certain serological investigations carried out on a rabbit fed with the embryonated eggs of Ascaris megalocephala.

2. By means of the complement-fixation test it has been shown that a definite immunity response occurs in the first five weeks of the infection.