1. Eleven new species of cercariae are described from China. Of this number ten have been obtained from species of bithynoid snails and one from Planorbis möllendorfi.

2. One of these species, Cercaria beatifica, a heterophyid larva, needs to be differentiated from the cercaria of Clonorchis sinensis, which develops experimentally in the same mollusc.

3. The forms described are much more common parasites of bithynoid snails than is the larva of Clonorchis.

4. Wherever it has been possible the larval characters, particularly those of the excretory system, have been utilised to show the relationship of the cercariae to adult groups.

In a previous paper (1924) I have discussed the reasons why a division of the order Eugregarinida into Septata (Lank.) and Haplocyta (Lank.) is preferable to Cephalina and Acephalina or Polycystidea and Monocystidea. The object of the present paper is to review the genera of Haplocyte gregarines so far described, and to attempt a classification of the same into families.

In previous papers one of us (Keilin, 1915, 1917) has shown that among cyclorrhaphous dipterous larvae there is a remarkable correlation between the anatomical structure of the larvae and their mode of life. Although the mode of life of the larvae is in correlation with such anatomical features as thickness and hardness of the body-wall, the development of sensory organs on the head, and the structure of the alimentary canal, it is in the bucco-pharyngeal armature that the most obvious and important adaptations are to be found. The most important of these adaptations may be mentioned briefly. In certain cyclorrhaphous dipterous larvae the ventral wall of the basal sclerite of the bucco-pharyngeal armature has a number of longitudinal ridges projecting into the lumen of the pharynx. These ridges are usually Y-shaped at their free borders, and form a series of longitudinal channels in the ventral region of the pharynx. In other cyclorrhaphous dipterous larvae such ridges are absent and the ventral wall of the pharynx is smooth. This character allows the larvae to be divided into two groups—“all cyclorrhaphous dipterous larvae parasitic on the most diverse animals or on plants, as well as carnivorous larvae, and larvae which suck the blood of mammals, never have ridges in their pharynx; on the contrary, ridges are always present in saprophagous larvae” (Keilin, 1915). All the larvae which are devoid of ridges and are either parasitic, carnivorous, pass their whole life in the uterus of the female, or are phytophagous, may be united into the group of biontophagous; all larvae which have ridges are saprophagous.

Of all the commoner families of parasitic Copepoda, the Chondracanthidae has received perhaps the least systematic notice.

In a recent paper (1927) Leigh-Sharpe and I divided the Chondracanthidae into two sub-families, the Lernentominae and the Chondracanthinae, and gave a full description and commentary on the three genera which form the Lernentominae. The present paper is an attempt to summarise our knowledge of the remaining sub-family, and introduces five genera and one species new to science.

Ten larval Cestodes, eight from aquatic and two from terrestrial hosts, have been described. Of these, eight species are new, and in the case of Cysticercus “A,” both larva and adult are new. By direct animal experiment the life-histories of three forms have been definitely established, while by careful comparison of the hooks of the scolex two larvae have been related to known adults. Cysticercus Hymenolepidis setigerae, previously recorded only from Copepods, is described here from an Ostracod host.

Effect of the parasite on the host. It is difficult to estimate the effect of Cestode larvae parasitic in Invertebrata, especially in the smaller forms such as the Entomostraca. In the latter, I have found that the intestine and musculature suffer most, and that the reproductive organs remain unattacked. The same remarks also hold for Oligochaete hosts. Dady (1901) considers that the influence of these cystic stages on their hosts is considerable, particularly on the musculature, sexual organs and intestine, and that death of the host may follow. Lindner (1921) states that little or no harm is done to Ostracods by the presence of tapeworm larvae, and also that the ovary of the female remains unattacked. The same author considers that, among the Entomostraca, destruction of the intestine occurs only in Copepods, the body cavity of Ostracods being so roomy that a harming of the intestine by growth of the parasite is unlikely. Schmidt (1894) finds that for Cysticercus Hymenolepidis anatinae, the host, Cypris ovata, was not particularly affected.

Of this Cestode I possess about a dozen adult specimens from one bird and nearly two dozen young specimens from another bird, preserved in the collection of the Wellcome Bureau and originally obtained, I believe, by Dr L. Sambon from the London Zoological Gardens. It is probably allied to, though certainly not identical with, the Ascometra vestita of Cholodkovsky (1912) obtained from the same host, viz. Houbara macqueeni. I have no information as to the locality from which the hosts were obtained in the present case. I shall describe the anatomy of this worm prior to discussing its systematic position.

The question of the mode of transmission of the Trichomonas of man from one host to another is of particular interest because no authentic encysted stage is known.

Many workers have attempted to infect “clean” animals by the mouth with active trichomonads from a variety of hosts, either from cultures or directly from faeces. The results are conflicting. Thus Escomel (1913) fed faeces containing Trichomonas intestinalis (= T. hominis) from a human case of dysentery to a rabbit, a guinea-pig, a cat, and a dog, and obtained a positive result in the dog. He does not, however, state that he examined his animals, to find whether they had any similar natural infections, before performing his experiments. Boyd (1919) fed a culture of Trichomonas intestinalis (= T. hominis), mixed with milk, to a white rat and infected it. He had previously examined this rat and one used as a control and found them to be free from this flagellate. He was, himself, uncertain whether the infection was due to the free flagellates or their “cysts“ which he says were present in the culture. The cysts figured by him (Plate XI, figs. 7–17) appear to be rounded-up and degenerating trichomonads. Pringault (1920) fed Trichomonas intestinalis (= T. hominis) to a cat, a guinea-pig, a rabbit, and a white rat, without establishing an infection in any one of them.

A short historical account is given of the writings on flea larvae since the seventeenth century. The larvae have been known since Leeuwenhoek tried to breed them in 1683.

The life-history of Ceratophyllus wickhami is described. The fleas were bred in the laboratory on a grey squirrel. Three larval instars occur, and the life-history occupies, on an average, 6 weeks under experimental conditions.

The external anatomy of the third instar larva of C. wickhami is described. The mouth parts are generalised and suggest the condition of a primitive insect. The tracheal system is composed of a double longitudinal trunk on each side, with spiracles on the prothorax, metathorax and first eight abdominal segments.

The first and second instar of C. wickhami are similar to the third instar larvae, except in size and the presence of a hatching spine in the first stage.

The larva of Ceratophyllus fasciatus is mentioned. The importance of the species lies in the presence of completely separated galea and lacinia.

Larvae of Xenopsylla cheopis and X. astia are briefly described. The larvae of the two species are practically identical except for the shape of the mandibles.

The disease “Heart-water” of sheep, cattle and goats has been recognised in South Africa for upwards of 70 years, and as long ago as 1902 Lounsbury demonstrated the part played by the common bont-tick, Amblyomma hebraeum Koch 1844, in its transmission. The virus of the disease does not pass through the egg of the tick, but a tick infected in the larval or the nymphal stages is capable of transmitting the infection when feeding as a nymph or adult respectively. Further, according to Lounsbury, a tick infected as a larva may feed upon an insusceptible host during the nymphal stage, and still remain infective until the final feed as an adult.

The advantages and difficulties of various methods employed in the study of larval trematodes are discussed. The desirability of using living specimens rather than fixed material and of studying mature, normally emerged cercariae rather than those obtained by crushing the host is noted. Intra-vitam staining with neutral red is recommended to demonstrate the form and reaction of the secretory granules in gland cells. Knowledge of the details of the excretory system is of major importance in both theoretical and experimental work.