William W. Cort

A Study of the Escape of Cercariae from Their Snail Hosts

During the summer of 1920 while studying at the University of Michigan Biological Station at Douglas Lake, Michigan, I undertook a series of experiments on the escape of cercariae from their snail hosts. The object of these experiments was to determine the numbers of cercariae escaping from snails infested with trematodes and the times at which these cercariae made their escape. Four species of cercariae were studied, viz.: (1) Cercaria elephantis Cort (1917), a schistosome cercaria with eyespots from Planorbis trivolvis Say; (2) Cercaria emarginatae Cort (1917), a forked tailed cercaria with a pharynx from Lymnaea enmarginata angulata Sowerby; (3) an undetermined echinostome cercaria from Physa ancillaria parkeri Currier, and (4) an undetermined stylet cercaria from the same host. Most of the studies were made on the escape of Cercaria elephantis from Planorbis trivolvis, the small number of experiments on the other three species of cercariae being carried out near the end of the work to obtain some comparative data. In order to determine which snails were infested with the cercariae the following simple method was used: A collection of about 100 specimens of a given species of snail would be brought into the laboratory late in the afternoon. These snails were then divided into groups of four or five and the groups placed in separate wide mouthed six or eight ounce bottles about one-third full of water. The next morning some of the water from these bottles was examined in a watch glass under a dissecting microscope. If any cercariae had escaped they could be easily seen and the groups containing infested snails determined. These groups were then redivided, only one snail being placed in a bottle, and by further examinations the individual snails infested were determined. Examination of the water from negative groups was repeated late in the afternoon to catch any cercariae which escaped only in the daytime. When a high percentage of snails were infested no preliminary division was made, the snails being immediately placed in separate bottles. With the infested snails isolated it was possible to pour off the water from around them at definite intervals and count the cercariae given off during certain definite periods. A new supply of water was

Two new species of strigeid cercariae from the Douglas Lake Region, Michigan.

Studies on the cercariae and life cycles of strigeids have been reported from the University of Michigan Biological Station since I9I7 when Cort (1917) described two species of cercariae of this group, Cercaria douglasi and C. emarginatae. These species were described very briefly and not figured in detail except for the excretory systems. One of them, C. douglasi, was redescribed by Cort and Brooks (1928) and was later shown by Van Haitsma (1931 a) to be the cercaria of Cotylurus flabelliformis (Faust, I917). The other, C. emarginatae, is now identified as the cercaria of Cotylurus comntmunis (Hughes, 1928) (see La Rue, 1932). Van Haitsma (I930) obtained the cercaria of this species by experimental infection of the snail intermediate host. While there are certain discrepancies between the two descriptions, it seems practically certain that they both refer to the same species of cercaria. Miller (1923 and 1926) described three more species of strigeid cercariae from this region, Cercaria burti, C. tenuis, and C. chrysenterica; and later Cort and Brooks (1928) added seven more, C. longifurca, C. flexicauda, C. laruei, C. nodicella, C. marcianae, C. bessiae, and C. physae. C. flexicauda has been shown by Van Haitsma (I93I b) to develop into Diplostomum flexicauddum (Cort and Brooks, 1928). The recent studies of the Hunters (I933 and 1935) and Krull (1932 and 1934) have shown that Cercaria bessiae is the larval stage of Crassiphialia ambloplites (Hughes, I927). Certain information on further development is also known for others of these species of cercariae. C. burti closely resembles the cercaria of Apatamon gracilis (Rud.), the life cycle of which was worked out by Szidat (1929 and I93I). C. chrysenterica penetrates into certain species of fish and appears to become localized in the eye (Cort and Brooks, 1928). C. laruei also penetrates into certain species of fish and develops into a diplostomulum in the lens of the eye (Cort and Brooks, 1928). C. marcianae was shown by Cort and Brooks (I.c.) to penetrate into tadpoles and develop into Agamodistomum marcianae (La Rue, 1917). This species appears to be closely related to Alaria mustelae Bosma, 1931, the life history of which was worked out by Bosma (193I and I934).

THE EARLY DEVELOPMENT OF THE DAUGHTER SPOROCYSTS OF THE STRIGEOIDEA (TREMATODA)

appear to be differences between species. However, there are rarely fewer than 12 or more than 30 in each sporocyst. The largest embryos of the germinal masses are at the ends and the production of free embryos in both mother and daughter sporocysts appears to be accomplished only by their breaking off. They are replaced by the growth of other embryos which in their turn separate from the masses; new embryos appear to be constantly formed from the germinal cells which persist in the germinal masses and multiply throughout the reproductive life of the germinal sacs. This mechanism for the multiplication of germinal cells in the strigeoid group produces in the mother sporocyst a sufficient number of daughters to fill completely the digestive gland of the snail host, and in the daughters the enormous numbers of cercariae which escape during the life of an infection. Up to the pres

A New Human Trematode from Japan.

In the last ten years the Japanese have contributed very greatly to the knowledge of the trematodes of man. Some of this work has found its way into other languages in translations and abstracts. There are, however, many important investigations on the trematodes of man in Japan which are either entirely unknown outside of Japan or are known only in very brief and inadequate abstracts. We have been working for some time on a monograph on the trematodes of man. In connection with this work one of us (Yokogawa) has brought into English the recent Japanese investigations on this group. In this connection it was surprising to discover that a new human trematode of more than incidental occurrence, Heterophyes nocens Onji and Nishio, which had been described in 1915 (Onji, 1915, and Onji and Nishio, 1915) was entirely unknown outside of Japan. The work of these authors on this species gains additional importance on account of the fact that they have not only described the adult but have solved part of its life history and shown the method of infection of the human host. Their work is the first light that we have on the life history or method of entrance into the final host of any member of the genus Heterophyes. The following account of Heterophyes nocens is taken from the Japanese papers cited above. In 1910, Onji, a physician in a district of the Yamaguchi province (in the southwestern part of the main island of Japan) reported before the medical society of the province that he had found a peculiar type of trematode egg in a number of fecal examinations which he had made in his district. At this time he thought that the eggs might be those of the human liver fluke. Later he came to the conclusion that he had found a new human fluke and reported his findings in 1915, including an account of an encysted larval trematode in the muscles of a fresh water fish, Mugil cephalus, which he thought belonged to his new adult trematode, since this fish was frequently eaten raw by the people of the district. Later in the same year, in collaboration with Nishio, he described this trematode as a new species under the name Heterophyes nocens and gave an account of certain stages of its life history and of its method of entrance into man.

Studies on a microsporidian hyperparasite of strigeoid trematodes. II. Experimental transmission.

In the first paper of this series (Cort, Hussey and Ameel, 1960) we reported on investigations made during the summers of 1956, 1957, and 1958 on the prevalence and effect on the parasitized larval trematodes of a microsporidian hyperparasite of the genus Nosema, which is found in a large proportion of the strigeoid infections in the snails of the region around the University of Michigan Biological Station near the northern tip of the southern peninsula of Michigan. We also carried out series of experiments designed to solve some of the problems involved in the transmission of this species of Nosema. As in the first paper of this series, we will refer to this microsporidian hyperparasite as Nosema sp. and will not try to determine its exact specific relationships until more details of its life cycle have been worked out. In a preliminary experiment done during the summer of 1956 we tried to find out what happened to the spores of Nosema that were ingested by snails that harbored no trematode infections. Several small laboratory raised juveniles of Physa parkeri were placed in small dishes with enormous numbers of the spores of Nosema sp. They were seen to ingest the spores during the period they were in the dishes with them, so that very large numbers must have entered their digestive tracts. Examinations were made of 2 of these snails, one after 36 hours and the other after 72 hours. Although their tissues were carefully teased apart and examined under the high power of the microscope, no trace of the spores or of anything recognizable as developmental stages of the microsporidian was found. Two more of the snails were killed and fixed for sectioning, one at 36 and the other at 72 hours after exposure to the spores. In examination under high power of the microscope of sections of the whole bodies of these snails no trace of the spores or of anything recognizable as a stage of Nosema sp. was found. Rather late in the summer of 1956 an experiment was performed in which snails harboring larval strigeoids were exposed to very large numbers of the spores of Nosema sp. During the summer of 1957 four more experiments of this same type were carried out, in which it was determined how long it took experimental infections to produce mature spores. In one of these experiments it was demonstrated that the metacercariae (tetracotyles) of Cotylurus flabelliformis could be infected with Nosema sp. Two further experiments in the summer of 1958 showed that the spores could survive at least a year and that several different species of larval strigeoids could be hyperinfected experimentally with this microsporidian.

Further studies on the development of the sporocyst stages of plagiorchiid trematodes.

As a part of a program of investigations on the embryology of the digenetic trematodes, a study was made, during the summer of 1941 (Cort and Olivier, 1943b), at the University of Michigan Biological Station of the development of the sporocyst stages of a common plagiorchiid, Plagiorchis muris Tanabe, 1922. The material was obtained from immature natural infections in juveniles and adults of Stagnicola emarginata angulata (Sowerby) from a beach where a very high incidence of P. muris was known to occur. Only late stages of the mother sporocyst of P. muris were found. They are entirely different from any mother sporocysts previously described for this group. They are oval, irregularly disc-shaped masses, 0.5 to 1.7 mm in largest diameter, which are made up of large numbers (about 300 to 500) of closely packed daughter sporocyst embryos (I.c., Figs. 1 and 2). These masses are discrete structures firmly attached to the outside of the snails intestine, and may occur anywhere along its whole length. Each daughter sporocyst in the mass is surrounded by an outer coat of irregular cells. In fact, the mother sporocyst, at this stage, has no special outer wall, but is composed entirely of the daughters, which appear to be held together by the cells of their outer coats which form the matrix of the mass. This outer coat of the daughter sporocysts persists throughout their whole life. It has been described for mature daughter sporocysts of a number of species of plagiorchiids, and has been called the paletot. In the largest mother sporocysts the daughters are elongate and mobile when freed. Each contains cercarial embryos of different stages of development, and a single large discrete germ-mass (l.c., Fig. 13). It was found that when the mother sporocysts are located on those portions of the snails intestine in front of the digestive gland, the daughters break away from the mother and migrate to the digestive gland and other organs of the snail, leaving behind no trace of the mother. When the daughter sporocysts cease migrating they become firmly attached to the snails tissue, grow thicker and appear crowded with cercarial embryos. Soon, thickened areas of the true sporocyst wall appear at their ends, which frequently

Variations in infections of Diplostomum flexicaudum (Cort and Brooks, 1928) in snail intermediate hosts of different sizes.

Diplostomum flexicaudun (Cort and Brooks, 1928) is by far the commonest trematode species in the snails of the Douglas Lake region of northern Michigan. Its cercaria was described by Cort and Brooks (1928) and its life cycle was worked out experimentally by van Haitsma (1931). The metacercaria was described by Hughes and Berkhaut (1929) as Diplostomulum gigas from the lens of the eyes of the common sucker, Catostomus commersonnii. The adult is a parasite in the small intestine of the herring gull, Larus argentatus. The cercaria of this species is very common in the beach snails, Stagnicola emarginata angulata (Sowerby) and S. e. canadensis (Sowerby). It has also been found in the Douglas Lake region in Lymnaea stagnalis appressa Say, L. s. perampla Walker, Stagnicola exilis (Lea), Stagnicola palustris elodes (Say) and Fossaria abrussa (Say). Since its snail hosts vary so greatly in size, D. flexicaudum is ideal for the study of the adaptation of a trematode species to intermediate hosts of different sizes. The cercariae of this species are very characteristic in structure and behavior, and there is not the slightest evidence that we have been dealing in this study with more than one species or with varieties of a species. Measurements of cercariae did not show size differences related to the size of the intermediate hosts. The slight variations that we found in the different lots, which were measured, appeared to be due to differences in the killing procedures, since some lots from a large host were slightly smaller than others from much smaller ones.

Development of the mother sporocyst and rediae of Paragonimus kellicotti Ward, 1908.

Ameel (1934) worked out the life cycle of Paragonimus kellicotti in experimental infections of Pomatiopsis lapidaria. Later, Chen (1937) made a study of the germ cell cycle of this species in which she followed the germinal cells in sections through all the stages of development. This was the most complete study of its type that had been made up to that time, and gave strong support to the theory that development in the germinal sacs of the digenetic trematodes is a germinal lineage. She determined that the diploid number of chromosomes was 16 and found no reduction in the divisions of germinal cells in the germinal sacs. This observation has been confirmed in the present study. During the summers of 1948, 1949 and 1950 at the University of Michigan Biological Station we studied the development of the mother sporocyst and rediae of P. kellicotti and traced the cells of the germinal line in living material. Our studies confirm most of the observations of Ameel and Chen on these stages and add new information on certain phases of their development. The study of living material gave a much clearer picture of the morphological relations of the stages in germinal development than could be obtained from sections.

Larval trematode infection in juveniles and adults of Physa parken Currier.

Studies on the ecological relations of larval trematode infections in Stagnicola emarginata angulata (Sowerby) have been carried out during the last five summers at the University of Michigan Biological Station (see Cort, McMullen, and Brackett, 1937; Cort, McMullen, Olivier, and Brackett, 1940a). One of the most interesting parts of this work has been the comparison of infections in juvenile and adult snails. In the summers of 1938 and 1939 the ecological studies were extended to include the larval trematode infections of juveniles and adults of Physa parkeri Currier, a very large beach snail, and, in addition, its life cycle was worked out. Two preliminary publications have already been made on certain phases of this work (Cort, McMullen, and Olivier, 1938; Cort, McMullen, Olivier, and Brackett, 1940b). In this paper all the data obtained on the life cycle of P. parkeri and on its larval trematode infections are presented.

An experimental test of the life cycle described for Cotylurus communis (Hughes).

On the basis of experiments conducted in 1926 and 1927, Van Haitsma (1930) presented a description of the life cycle of a strigeid trematode designated by him as Cotylurus michiganensis. Later, La Rue (1932) pointed out that the correct name for the species is Cotylurus communis (Hughes, 1928). Van Haitsma demonstrated by feeding experiments that strigeid metacercariae identified as Tetracotyle communis Hughes found on and about the hearts of common suckers, Catostomus commersonnii (Lacepede), became adults of C. communis in the bursa Fabricii of the herring gull, Larus argentatus (Pont.). He then collected eggs from the bursae of the experimentally infected gulls and allowed them to embryonate. Seven species of snails were exposed to the miracidia which developed. One of the snails, a specimen of Lymnaea emarginata angulata Sowerby (= Stagnicola emarginata angulata), produced large numbers of strigeid cercariae forty days later. This snail had been collected from Douglas Lake and kept in the laboratory three weeks before it was exposed to the miracidia. The intestine of the gull, from which the eggs used in this particular experiment were obtained, harbored two specimens of a species of Diplostomum. The bursa contained many specimens of C. communis. Van Haitsma concluded that the single snail mentioned had been infected experimentally and that the cercariae belonged to C. communis. He stated that similar experiments conducted in 1928 yielded the same results but he did not describe these experiments. Cort and Brackett (1937) considered the cercariae obtained by Van Haitsma to be identical with C. emarginatae Cort, 1917 which was described from the same host in the Douglas Lake Region. Szidat (1931, p. 166) questioned the validity of the life cycle of C. communis as outlined above. He believed that the location of the penetration glands in the cercariae of members of the same genus should be similar. Since the penetration glands of Van Haitsmas cercaria lie behind the ventral sucker while those of the cercaria of C. cornutus are