1. The relation of host-parasite specificity to age resistance.

Conclusive demonstration of a substantial resistance developing concomitantly with the ageing of the host has been made with reference to four helminths, namely: Ancylostoma duodenale and A. caninum in the dog and Ascaridia lineata and Syngamus trachea in the chicken. A. duodenale in the dog and S. trachea in the chicken are manifestly in abnormal hosts. In contrast with this, it is to be noted that the normal hosts of these parasites, respectively man and the turkey, exhibit no appreciable age resistance. With regard to the remaining two cases, those of Ancylostoma caninum and Ascaridia lineata, the association of age resistance and a specific host-parasite mal-adjustment is not nearly so clear. In nature, both parasites enjoy a polyxenous distribution, neither having acquired the need for strictly specific host conditions for development. As a result of recent researches with A. caninum it has been brought out that the species is comprised of strains better adapted to one species of host than to another, and it has been demonstrated that a higher degree of age resistance is exhibited in the host parasitised with a foreign strain. Although no information on the matter is available, it is quite possible that age resistance will be found to be less markedly expressed in other species or races of suitable hosts, when the bionomics of both Ancylostoma caninum and Ascaridia lineata are further investigated.

The species described in this paper were contained in a collection of parasitic worms made in Java by Dr. E. Jacobson, and sent to Prof. G. H. F. Nuttall, F.R.S., by whom the material was kindly forwarded to the writer for determination.

Jameson (1926) has described from the caeca of cattle a ciliate belonging to the family Isotrichidae to which he has given the name Buxtonella sulcata. The most prominent character of this ciliate is a dorsal ridge running in a wide sweeping curve from one end of the body to the other with a groove running down the middle. Other characters of importance are a peculiar indentation near the mouth and the not uncommon occurrence of the macronucleus in two separate rounded portions. Roundish oval cysts of this ciliate, 80 to 100 microns in length by 60 to 80 microns in width, were also found by him.

There can be no doubt that reasoning by analogy contributes enormously to the progress of Science by suggesting the hypotheses which form the basis of experimental investigation. But although it is useful, it is also dangerous and it is not infrequently the source of plausible but inaccurate and misleading catchwords.

In response to my request for specimens of blood from Lepidosiren paradoxa, Dr G. S. Carter and Mr L. C. Beadle very kindly made a large number of dried films from 34 fish caught between the dates Sept. 25th, 1926, and May 24th, 1927. These fish were caught in the same locality in the Gran Chaco from which came the infected specimen of my former note (Jepps, 1927), on the parasite which I called Haemogregarina lepidosirenis.

The anatomy, histology and digestive enzymes of the mid-intestine of the tsetse-fly have been investigated, and an attempt has been made to determine the functions of the various parts and to observe the changes to which they are subject during the digestion of blood.

Histologically the mid-gut of Glossina consists of three regions:

(i) An anterior segment of small, pale-staining, irregularly columnar cells, which comprises about half the total length of the mid-gut. The zone of giantcells containing bacteroids, which is very limited in extent, lies at about the middle of this region.

(ii) A middle segment of large, deeply staining cells, heaped together in the resting state, which is separated abruptly from the anterior segment.

(iii) A posterior segment, arising by gradual transition from the middle segment, composed of regular columnar cells.

After a meal the blood is concentrated by the removal of fluid in the anterior segment but it shows no other change in this region. The giant-cells are greatly flattened but they do not regularly discharge the bacteroids which they contain and there is no evidence that these organisms play any part in the digestion of blood. Their possible function has been discussed.

During digestion the cells in the middle segment contain globules of secretion, and vacuolated buds of cytoplasm are set free and disintegrate in the lumen. The blood shows an abrupt change on reaching this region; it turns black where it is in contact with the epithelium and amorphous masses of altered blood pigment are deposited.

In the posterior segment, the epithelial cells become greatly vacuolated later in digestion and are probably concerned chiefly in absorption.

The distribution of digestive enzymes agrees with these histological observations. The salivary glands and proventriculus contain no digestive enzymes, and the anterior and posterior segments of the mid-gut also are practically inactive. But the middle segment produces a very active tryptase which agrees in its pH-activity curve and other properties with the tryptase of the cockroach. A peptidase also is present but, except for a very weak amylase, enzymes acting upon carbohydrates are absent. The contents of the mid-gut are always slightly acid (about pH 6·5) and the tryptase present is well adapted to work at this reaction.

These findings have been contrasted with those in a non-blood-sucking muscid (Calliphora). Here the salivary glands secrete an active amylase and the mid-gut is rich in amylase, invertase and maltase, whereas the proteolytic enzymes are extremely weak.

Some observations have been made upon the tracheal supply to the walls of the gut. The epithelial cells of the middle segment have been shown to contain a very rich supply of intracellular tracheoles. These are usually difficult to make out in the resting cells but after a large meal the surface of the cells is ruptured and blood pigment enters the tracheoles and may extend to the sub-epithelial tracheoles and tracheae or even to quite large tracheal trunks. As the epithelial cells are flattened by the pressure of the meal, this pigment is set free in the lumen in the form of dark rods of haematin, which often bear a superficial resemblance to bacteria. The pigment in the deeper tubes appears to be slowly absorbed later. Intracellular tracheoles similar to these are present also in the mid-gut of Calliphora.

The proventriculus in Glossina is a complex and has always been a puzzling structure. It has been shown that it acts as a sphincter between the fore-gut and mid-gut and that it is responsible for the production of the peritrophic membrane. This membrane, which is composed of chitin but contains a small quantity of protein, is secreted in the form of a fluid by the ring of large epithelial cells at the base of the proventriculus. The fluid is pressed and condensed to form a uniform membrane by being drawn through the cleft between the wall of the proventriculus and the funnel-shaped invagination of the fore-gut.

The function of the peritrophic membrane has been discussed and it has been shown that it is freely permeable to digestive enzymes and to haemoglobin.

Small trematodes were observed by Dr G. S. Carter and Mr L. C. Beadle in the pericardiac cavities of Lepidosiren paradoxa caught in the swamp at Makthlawaiya in the Gran Chaco, during the months January to June 1927. The infected Lepidosiren varied in length from 20 to 34 ins., and constituted almost one-half of all the fish collected during that period. Dr Carter tells me that the infected fish appeared to be in perfect condition. Specimens from fifteen fish were preserved, and have been examined.

During trawling expeditions off the coast of Galway in 1926–1928 I examined 100 specimens of Solea vulgaris Quensel, of varying sizes, and found about 60 per cent. infected with the parasite herein described. Usually two to five parasites were found attached to an infected fish, but up to eight were discovered on a single host.