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.

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.

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.

Phthirus pubis is not known to serve as a vector of any infective disease.

The pathological effects of its parasitism on man are on the whole of slight degree. Some persons are more sensitive than others to its presence; on the one hand the louse may produce no reaction by its bite or it may be tolerated upon the person for years, on the other hand it may induce pruritus of a more or less severe character, especially in parts of the body that are most infested and where the skin is most delicate, i.e. about the genitalia, the axillae, and eyelids. All grades of pruritus are observable, scratching of which the individual is unconscious may occur, or in the severest cases the scratching goes on day and night, so that by day it may be evident to spectators and aid in diagnosis. The scratching may only begin from the moment when a person realizes that he is infested.

The pale bluish-grey maculae are a specific sign of the presence of Phthirus upon man, but they are not always present. They are induced by the adult louse, are few in number, painless, deep seated, do not disappear on pressure, and measure 0·2–3 cm. in diameter. The maculae mark the site of the insect's bite; they appear 8–24 hours after the louse commences to feed on the spot affected. The bites of P. humanus do not produce maculae caeruleae. The salivary glands of Phthirus and Pediculus give correspondingly positive and negative results when inoculated subcutaneously. The maculae disappear usually within a week after the removal of the offending parasite, and they disappear more rapidly when the skin is irritated. The nature of the maculae remains to be determined, they may be due to extravasated and altered blood.

Apart from the maculae, Phthirus, like P. humanus, fleas and mosquitoes, may cause a febrile condition owing to skin irritation, although this appears to be rare; with the removal of the lice, the fever promptly subsides. Papular eruptions and eczematous inflammation may supervene upon the presence of the louse, but they appear to be largely due to the added insult of scratching the infested skin. The melanism is of non-specific origin, being due to continued scratching which leads to hyperaemia, pigmentation and desquamation. The lesions induced thereby may lead to secondary skin infection with pyogenic bacteria.

Although Phthirus pubis occurs in Europe, Asia, Africa, North America and Australia, and is found on negroes as well as whites, as herein recorded, further data are required relating to the geographical distribution of the species and the races of man it infests.

The crab-louse occurs less frequently on man than does Pediculus and appears to be parasitic chiefly on persons leading an active sexual life. It has, so far, only been twice recorded on another host than man, i.e. the dog.

Whilst the crab-louse is usually found confined to the pubic and perianal region, it frequently spreads upward upon the abdomen and breast, and may infest the axillae severely, or it may spread downward along the thighs. Generalized infestation is rare, in such cases only the hands and feet are spared, although the head and neck are as a rule also free from parasites. The crab-louse has occasionally been found localized upon the head in infants, being either confined entirely to the eyelids, the nits being encountered on the eyelashes, or occurring also upon the eyebrows and hairy portions of the scalp. In adults, the parasite occurs much more rarely in these situations; a case is recorded of an Arab in whom, in addition, the beard and moustache were infested. Such cases are rare in children, very rare indeed in adults.

Apparently the head is so rarely infested because it is less suitable as a habitat. As Waldeyer points out, the head is probably not a suitable habitat because the scalp-hairs are crowded close together and finer than on the pubis and in the axillae. The majority of the active stages are found clinging to two hairs on the regions of the body they most infest, and where the hairs are sparser than on the head. In these parts, the hairs are 2 mm. or more apart. The reach between the extended two hind leg-pairs of the adult insect is about 2 mm., these legs being the ones that are used for grasping the hair.

Whilst infestation usually takes place through coitus, there are many exceptions to the rule as proved by the occurrence of crab-lice on infants. The latter may become infested by their parents or other adults. The insect may pass from one infant to another through their sharing a cradle, or it may pass from soldier to soldier when crowding occurs in barracks or billets. Scratching of their persons by infested individuals no doubt aids in the spread of the parasite upon them and the dissemination of the insect further afield. Both the act of coitus and scratching promote the shedding of hairs, and these, when bearing nits or young larvae, may be of considerable importance in disseminating the parasite. Such hairs, and no doubt occasionally active stages of the louse, are shed on clothing, bedding, the seat of the privy, etc., and readily become entangled with the pubic or other hair of clean persons who may come in contact therewith. A detached louse promptly clings to any hair with which it comes in contact. Therefore, whilst Pthirus is commonly conveyed directly, it may also be acquired indirectly. It is a helpless creature when removed from the hair to which it clings continuously upon the body, where it moves about by shifting from hair to hair; it is therefore much more likely to be conveyed passively from host to host than is Pediculus.

Unless disturbed, the parasite remains confined throughout its life to a limited area upon the host's skin as I have demonstrated experimentally.

The females appear to preponderate over the males in number when upon the host, their relative proportion being about 3:2. In copulation, the male does not seize the female as in Pediculus; he seizes the hairs to which the female clings, using these as a support. Oviposition occurs as in Pediculus. A female that was raised experimentally, laid up to three eggs per day, laying a total of 26 eggs. The hatching period (on the leg) lasted 7–8 days. Like Pediculus, the crab-louse passes through three moults (there being as many larval stages) before it attains sexual maturity. When the adults are about 10 days old they exhibit to a well-marked degree the greenish coloration of the fat-body which is seen by transparency at the sides of the abdomen in the living insect. The life-cycle, from egg to egg, is completed in 22–27 days (on the leg).

Young unfed larvae usually die within ten hours of emergence. When removed from man, they survive longer at 16–20° C. than at 30° C. and die much more rapidly in a dry than in a moist atmosphere; none of the numerous lice of all stages that were tested were found to survive up to 42½ hours, when maintained under different conditions. Living on man a male survived 22 days, a female 17 days, but the insects can doubtless live longer.

The feeding habits of Pthirus recall those of Ixodidae in being practically continuous, and, like Boophilus, they stop feeding to moult upon the host. After moulting they promptly proceed to feed again, shifting but slightly to a fresh feeding ground. On emerging, the young larva clasps a single hair and feeds at its base, and after a day or more it may clasp two hairs as the later stages do usually. The habit of continuous feeding explains why the insects die so quickly when removed from the host.

Whilst a brief exposure to light renders the insects restless but does not cause them to wander away whilst feeding, a warm body brought within their sphere promptly produces great activity in Pthirus when it is removed from man.

The gregarine herein described lives in the larvae of the moth Endrosis fenestrella Stain. which I found in the nests of the house-martin (Chelidon urbica Linn.)