The usual host of this new parasite, Helicosporidium parasiticum, is the larva of a Ceratopogonid, Dasyhelea obscura Winnertz, which lives in the decomposed sap filling the wounds of trees—elm and horse-chestnut. All the material used for this study was derived from the wounds of two trees only: (1) an elm tree standing on the Caius College ground at Newnham, facing Church Rate Walk, and (2) a horse-chestnut standing between the School of Agriculture and Downing College.

Warburton (1920) has recently published a survey of our present knowledge of the mites of the genus Sarcoptes; from his survey it is clear that in spite of all the work that has been done upon these mites, much of it by most pains-taking anatomists, we are still without an accurate knowledge of the anatomy of any one species of Sarcoptes. This is, in part at any rate, due to the fact that some of the best work was done, by Robin and others, between 1860 and 1865, at a time when microscopes and technique were by no means as highly developed as they now are.

Our knowledge of the systematics of the genus Sarcoptes is in an unsatisfactory state. About a score of species have been admitted, many of which have never been seen since they were described. Many of the descriptions date from between sixty and seventy years ago, a period when microscopes and microscopical technique were barely adequate to cope with so difficult a genus as Sarcoptes is. In order to provide a basis for further study of Sarcoptes, Warburton (1920) published a critical survey of our knowledge regarding these mites. His review of the literature has proved invaluable to myself, and is the foundation on which I have worked. In the foregoing paper (pp. 114–145) I published an only too lengthy description of the Sarcoptes of the horse, finding that a full description of one form was necessary as a basis for comparison. Since we do not know what anatomical points are of systematic importance, attention was given to all details of the external anatomy.

Among the recent advances in our knowledge of the aetiology of human diseases, one of the most interesting is the discovery of a new group of somewhat problematic organisms which have been grouped together under the generic name Rickettsia. Up to now their presence has been shown, more or less satisfactorily, to be associated with three types of human fevers, viz. typhus, trench fever and possibly Rocky Mountain spotted fever, and in the last few years a certain number of papers have appeared on this subject, the most comprehensive of which are those published by the British War Office Committee and the American Commission on trench fever. In addition it has been suggested that many other human diseases are caused by these organisms, but the evidence is so very unsatisfactory that it may be disregarded.

There are three species of true parasites in the genus Aphelenchus, viz.: A. fragariae R.B., 1891, A. olesistus R.B., 1893, and A. phyllophagus n.sp. A. ormerodis R.B., 1891, is a doubtful species, possibly a young form of A. fragariae (Marcinowski, 1908).

A. fragariae causes the disease strawberry bunch; A. olesistus and phyllophagus cause leaf disease in flowering plants and ferns. The diseases are widely distributed and of considerable economic importance.

From the study of the anatomy of the three species certain points of general importance emerge: (1) The salivary glands of Aphelenchus represent the oesophageal glands of other nematode genera, which have separated from the body of the oesophagus. This separation causes the reduction in size of the posterior portion of the oesophagus, and its lack of clear demarcation from the intestine. (2) The excretory organ of Aphelenchus, and of some other nematode genera, originates from the left lateral line. (3) The vaginae are ectodermal organs distinct in origin from the gonads proper.

The life cycle of the parasitic Aphelenchi is divided into three definite stages: (A) That of residence in the definitive habitat, which for A. fragariae is in the stem and leaves of the strawberry plant, for A. olesistus and phyllophagus in the mesophyll spaces of the leaves of many plants. Nutrition and reproduction are most actively carried on in this situation, several or many generations succeeding each other, and here the pathological effects are manifest. (B) The resting stage in the soil. The worms reach the soil in fallen leaves, and the adults survive in a partially dried, dormant condition on the surface of the soil for prolonged periods. (C) The stage of immigration into the host plant. When revived by moisture the Aphelenchi may be attracted by a suitable plant, and wanders on to it; they may live as ectoparasites in the leaf axils, breeding in this situation, the larvae migrating to the definitive habitat; or they may enter the definitive habitat direct. They do not traverse the tissues of the stem during immigration. A. olesistus and phyllophagus enter the mesophyll spaces through the stomata.

It should be noted that this life cycle is that of the race, not of the individual. It comprises many generations, and in this respect resembles that of Tylenchus dipsaci and differs from that of T. tritici, which includes one generation only.

In the life of the individual Aphelenchus, embryonic development occupies not more than five days, embryonic and larval development not more than ten, and a complete generation not more than fourteen days.

High atmospheric humidity assists the migration of the worms, previous disease or injury of the host plant may do so. Some races of various species of plants are immune.

Treatment of the affected plants by chemical methods is of limited applicability. Treatment by immersion in water at 50–52° C. for five minutes is recommended by Marcinowski (1908). Prophylaxis should be based on burning of infected plants and leaves, and on the avoidance of infected surface soil in the filling of pots and boxes.

In my previous paper (Parasitology, XI. 1919) on the life history of a Calliphorine fly, Melinda cognata Meig., the larvae of which live as parasites in a snail (Helicella virgata), I gave a complete account of all the Dipterous larvae feeding upon molluscs.

In Pediculus humanus are found two groups of excretory-accumulatory cells known as nephrocytes.

The one group, the peri-oesophageal, lies ventrally and consists of large cells aggregated usually in two masses about the oesophagus anterior to the reniform salivary glands.

The second group lies dorsally and consists of disseminated cell aggregates linked with the fat body.

The typical nephrocyte is a binucleate cell with granular protoplasm containing greenish droplets of varying size. The excretory function of the nephrocytes is demonstrated by intra-coelomic injection of ammonia-carmine. The latter, 24 hours after injection, is taken up by the nephrocytes which become red and filled with the carmine granules. These granules remain in the protoplasm of the nephrocytes throughout the life of the insect.

Similar cells exist in Mallophaga and have been wrongly described by some authors as salivary glands.

The two groups of nephrocytes, described by us in Pediculus, occur in other insects, but the dorsal group in the latter forms usually two chains of cells (known as pericardial cells) lying on either side of the heart.