“Man, the chief lord of other animals, is, notwithstanding his power, the food and ordinary abode of this vermin, who riots in his blood, and sometimes colonizes his body with innumerable and detestable offspring. When such a mean, disgustful insect as this, can raise legions sufficient to render thee loathsome, and even to destroy thee, O man! where is thy boasted greatness?” Barbut (1781, p. 328).

Some years ago, in endeavouring to obtain various stages in the life history of the liver-fluke in South Africa for teaching purposes, I found that, although the parasite is fairly common on some farms in this country, the intermediate host had not yet been definitely ascertained. In Europe, as is well known, the intermediate host is the fresh-water snail Limnaea truncatula, but this species has not been recorded with certainty from South Africa. The commonest fresh-water snail here is Physa (Isidora) tropica and a preliminary examination of a few specimens of this mollusc from Muizenberg Lake revealed abundant stages of fluke very closely resembling those of the liver-fluke. Additional specimens procured from the neighbourhood also afforded abundant material of the same stages. The only difficulty in accepting this as a solution of the problem was that, though placed in the most favourable conditions, the free swimming form (cercaria) was never observed to encyst.

This bibliography—the first of the kind hitherto published—deals with zoological and medical publications relating to the two species of lice (Pediculits humanus Linn. 1758 and Phthirus pubis Leach, 1815) infesting man, the part they play in pathology, the prophylactic measures and means of destruction employed against lice, their anatomy and biology and other matters pertaining to them.

In previous papers the migration of Ascaris lumbricoides in the rat and mouse has been traced to the point at which the larvae of the worm reach the trachea of these animals on the 8th day after infection. As the result of further experiments it is now possible to trace the migration through a further stage, namely that occurring from the 9th to the 15th days after infection. My experiments have not as yet been carried beyond the 15th day and in the following summary the statement that larvae occur in a given situation up to the 15th day is not intended to imply that they do not persist therein after that date.

The term Pediculosis denotes the pathological condition brought about by infestation with Pediculus humanus, either the head or the body or both of these being affected. Pediculosis may lead directly or indirectly to secondary infections of various kinds. P. humanus (corporis and capitis) has been shown to be the carrier of two important diseases affecting man, namely typhus and relapsing fever.

I. In the following three experiments it will be shown that ripe eggs of Ascaris suilla can hatch in the intestine of the pig, that the larvae issuing from these eggs enter the body of the pig and pursue the same course through the body as in the rat and mouse. They have been found in the lung of the pig from the 6th to the 8th day and in the trachea on the 8th day. Dead larvae have been found in the faeces of the pig on the 11th day after infection (Expt. 6).

1. Entamoeba histolytica Schaudinn (vel E. dysenteriae Councilman and Lafleur), the human dysentery amoeba, is a collective species. It comprises a number of distinct races, strains, or pure lines, distinguishable from one another by the size of the cysts which they produce.

2. How many such distinct races exist is still undetermined; but we have demonstrated the existence of at least five.

3. There is no evidence that the different races differ in their geographical distribution, or in any character save size.

4. These races remain constant in character within a given host; and the dimensions of the cysts are not determined by the action of the host upon the parasite, since two different races may coexist side by side in the same host.

5. Different values for the dimensions of the cysts are obtained when these are measured in different media. Taking the measurements of living cysts suspended in physiological saline solution as representing the true value, we find that the dimensions of cysts in iodine solution are approximately the same; but that cysts which have been suitably fixed, stained, and mounted in balsam, have undergone an apparent reduction of approximately 10% from the true diameter.

6. The most suitable fixative for E. histolytica cysts we have found to be a modification of Schaudinn's fluid. (Saturated aqueous solution of corrosive sublimate, two parts; absolute alcohol, one part; glacial acetic acid, 4–5%.) Cysts fixed in this fluid undergo no measurable shrinkage or swelling in the process.

7. The apparent loss of size observed in cysts which have been fixed, stained, and mounted in balsam, is due chiefly to the invisibility of the cyst wall in the mounting medium.

8. It follows from what has been said above (5 and 6) that to obtain an approximately true value for the dimensions of the living cysts from measurements made upon those suitably fixed, stained, and mounted in balsam, these measurements should be multiplied by the factor

9. We find no evidence of the existence of a sexual dimorphism in the cysts belonging to a single race, such as has been described by Mathis and Mercier.

10. Since cysts of E. histolytica may be found with all diameters from about 5μ. to about 20μ—a range overlapping or covering the dimensions of the cysts of the two common harmless intestinal amoebae of man (E. coli and E. nana) and of other cysts and cyst-like bodies occurring in human faeces—it is of considerable practical importance to recognize and distinguish the diverse races of E. histolytica which differ from one another in the dimensions of the cysts which they produce.

It is now becoming generally recognized that there are three different amoebae which may be present in the human bowel—the harmless Entamoeba coli(Lösch) Schaudinn and “Entamoeba” nana Wenyon and O'Connor, and the pathogenic E. histolytica Schaudinn. We have discussed these three species in an earlier paper (1917), and there noted their chief distinctive characters. In the present paper we shall describe a fourth and much less common species which we have recently encountered. It differs in some respects so conspicuously from the others that it appears to us necessary to place it not only in a new species, but even in a new genus. We propose therefore the new name Dientamoeb fragilis for our amoeba, but we can discuss the question of nomenclature most conveniently after we have described the organism itself. This, therefore, we shall now do.

In the following account of the biology of Pediculus humanus, I have sought to bring together all that could be gathered from the publications of the many authors who have occupied themselves more or less with these insects, adding here and there my own observations2. I have sought throughout to give credit where it is due and have not withheld criticism where it appeared necessary.

During the period 1913–16, I conducted a Survey of the Agricultural Zoology of the Aberystwyth Area, for which purpose the University authorities received a grant from the Board of Agriculture. The area thus examined comprised N. Cardiganshire and some adjacent borders of Montgomeryshire, roughly about 250 sq. miles. It falls into three main physical divisions:

I. The mountainous upland (or high plateau) of solid rocks frequently covered by a considerable depth of peat. The rock succession is only irregularly exposed in deep stream cuttings, while these valleys are occupied by boulder clay and river deposits.

II. The surface of the coastal plateau shows exposures of rock and of boulder clay. The clay is usually exposed and rarely covered with peat.

III. The fall line, which is intermediate. Here the boulder clay generally follows the valleys, and it is usually covered with a thin layer of peat. Much of this is readily cut through by small streams.

1. Certain flocks in the Aberystwyth Area having suffered considerably from Liver Rot, a Survey of the Agricultural Zoology was carried out between October, 1913, and October, 1916, special attention being given to the above disease and the bionomics of the host snail L. truncatula.

2. The Area comprised some 250 sq. miles of North Cardiganshire, etc., including Plynlymon, and falls into six natural divisions.

3. The presence of the disease has caused a number of local modifications in farm practice, designed to prevent, or mitigate losses. The disease was found to be on the wane within the Area, though endemic in certain places.

4. L. truncatula and L. peregra are the only abundant local species of the Genus. Considerable differences occur in the distribution of these two species. L. peregra was only studied in order to better understand the habits of L. truncatula. This latter species occurs in shallow water floored with Diatomaceous clayey mud, and is absent from soft mud, and peat. There is a marked coincidence between the distribution of clay and L. truncatula, and L. truncatula and Liver Rot.

5. The growth of the snails takes place throughout the year unless interrupted by drought or severe frost. Ova are deposited in March–April and hatch in May. The resulting snails, when they become adults, measure about 5 mm. in length. They in turn deposit ova during June–July. Drought usually limits further progress until the late Autumn, the snail being often very scarce during August–September.

6. Desiccation speedily kills L. truncatula. Local environmental conditions—rainfall, aspect, soil, vegetation, etc.—play a very important part in respect to its survival. Rainfall and its distribution through the year is the most important factor governing the increase, spread, and infectivity of the snail.

7. The ova masses dry rapidly and then resemble a hard inconspicuous scale. They resume their former size and shape when moisture returns, and many appear to be able to hatch subsequently, though the amount of desiccation they will withstand is not yet determined. In spite of prolonged droughts, L. truncatula re-appears, unless ditches, etc., have been cleaned out.

8. Roadside ditches are very frequently inhabited by infected snails and the drainage from them may be the cause of cases of rot.

9. Cercariae of D. hepaticum were obtained from L. truncatula in large numbers practically throughout the year; but not from snails measuring less than 4·5 mm. in length. Seventy cercariae were given off naturally, within 48 hours, from one specimen measuring 7 mm. in length. Heavily parasitized snails are adversely affected. A few cercariae, resembling those of D. hepaticum were obtained once from L. peregra.

The examination of a large number of individual lice in India, where the facilities for obtaining material are unusually good, has impressed me with the fact that the characters ordinarily given as differentiating corporis from capitis are all to a greater or less extent inconstant and unreliable. In general, however, capitis is more heavily chitinized and the chitin is of a darker colour with less smooth outlines. The size is very variable, but corporis has an easier life and is more often large.

The preparation of the fourth part of this paper was brought to an abrupt conclusion in November, 1915, by an illness which necessitated my return to a cooler climate. On that account the present part is incomplete, several new and interesting forms being left undescribed. These, together with the species described here, are deposited in the collection of the Australian Institute of Tropical Medicine.