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reproductive system of the trematodes and cestodes, and consists of a series of glandular structures surrounding the ootype, the tube connecting the oviduct to the uterus. These glands vary in number and arrangement in the different groups and species. This gland is particularly well developed in the liver fluke, F. hepatica, where its huge size has made it difficult to appreciate its relationship with the complicated system of ducts embedded in it. In consequence there has been a conflict in interpretation (Stephenson, 1947, Yosufzai, 1953) and the purpose of this paper has been to attempt some clarification.

Egg shell formation for Parastrigea mexicanus is described. Eleven histochemical tests were used to elucidate the process and to characterize the oogenotop. The egg is formed in the ootype (a region of high secretory activity); an oocyte and 12-17 vitelline cells are enclosed by a coalescence of vitelline shell globules. The vitelline cells also contribute copious amounts of glycogen and RNA. There was no direct histochemical evidence for Mehlis or uterine gland function; however, secretory functions were inferred from morphology and histochemical reactions. The basic mechanism of egg shell formation in P. mexicanus closely follows that of Fasciola, but it differs in some regards from the process described for the latter trematode. Helminthologists have studied egg shell formation in trematodes for 82 years now and their efforts have concentrated on widely separated taxa. In spite of this long period of time and the many investigators involved, only a few of the more than 100 families of digenetic trematodes have been studied in this regard. Our approach has been a comparative one and to date, we have completed studies on representatives from the following families: Fellodistomidae, Hasstilesiidae, Hemiuridae, Microphallidae, Notocotylidae, Pronocephalidae, and Syncoelidae. Fasciola hepatica has received the greatest attention; Haematoloechus has been studied in some detail also. Egg shell formation in the digenetic trematodes appears to follow a generalized mechanism which has been elucidated by a number of authors and reviewed by Smyth & Clegg (1959). Since that date, a number of papers have described variations on the central theme. Recently, Ebrahimzadeh (1966) studied 27 species (from ten families) on a comparative basis and he thus established a standard of excellence for students of egg shell formation to follow. During the course of other studies on parasites of the black-necked stilt, the author collected a large number of Parastrigea mexicanus Coil, 1955 (Strigeidae). Although the strigeoids have received much attention concerning their life cycles, it has been only recently that details of their morphology have been studied; many of these have emphasized the holdfast organ (Erasmus, 1968; and others). There are certain striking features of these worms which warrant further studythe large amount of tissue devoted to vitelline production, the extensive Mehlis gland tissue, the extremely large egg, and the nature of the uterus. P. mexicanus provides excellent material for study. The worm is short, compact, and easily handled; sectioned material can be prepared without difficulty. Microdissections are difficult, however, due to the muscular body wall and the heavy cuticle. This is the fifth report in our series of comparative studies on egg shell formation in digenetic trematodes. 1This investigation was supported by Public Health Service Grants AI 05145 and GM 12587. 2The author is indebted to Dr. Clarence Cottam and Mr. Caleb Glazener, both of the Welder Wildlife Foundation, for their help in providing facilities, equipment, and supplies for the early parts of this study. TRANS. AMER. MICROSC. Soc. 88(1): 127-135. 1969. This content downloaded from 207.46.13.21 on Tue, 27 Sep 2016 05:13:10 UTC All use subject to http://about.jstor.org/terms 128 TRANS. AMER. MICROSC. SOC., VOL. 88, NO. 1, JANUARY 1969

Observations on the functional anatomy of the ducts associated with Mehlis' gland were made in five species of echinostomes: Echinostoma malayanum, E. lindoense, E. audyi, Hypoderaeum dingeri, and a 43-spined, undescribed Echinoparyphium species. In all five species the structure of the Mehlis' gland complex is similar. There is an ovicapt in the form of a chamber which regulates the passage of the ova from the ovary into the oviduct. Ciliated epithelium lines the ovicapt, the oviduct distal to the ovicapt, and the proximal part of Laurer's canal. The movement of the cilia is directed toward Laurer's canal from which excess sperm and vitelline granules are discharged at the dorsal surface. Spermatozoa are stored in the uterine seminal receptacle in the absence of a seminal receptacle. The Mehlis' gland complex of echinostomes is best studied in living worms because the ducts and organs associated with it are not clearly visible in mounted specimens. Moreover, the gland region is often filled with eggs, making a detailed study extremely difficult or impossible. Descriptions are often incomplete and sometimes inaccurate. This paper reports observations on the Mehlis' gland complex in living specimens of five species (three genera) of echinostomes. MATERIALS AND METHODS The following species of echinostomes were studied: Echinostoma nmalayanum Leiper, 1911; E. lindoense Sandground and Bonne, 1940; E. audyi Lie and Umathevy, 1965; Hypoderaeum dingeri Lie, 1964; and an undescribed 43-spined Echinoparyphium species which will be named later when its complete life cycle is described. An unlimited supply of living worms was maintained in the laboratory for life-cycle studies. E. malayanum was obtained from rats, the other four species from pigeons and ducklings. Young adult worms with few eggs in the uterus were preferred. The Echinoparyphium species is the easiest to study since the worm is small and seldom contains many eggs; H. dingeri is the most difficult to examine due to thickness of the worm and density of Mehlis' gland. The living worm is examined dorsal side up in a drop of physiological saline under a cover slip. Received for publication 12 August 1964. * This work was supported by the University of California International Center for Medical Research and Training (The Hooper Foundation, San Francisco School of Medicine) with Research Grant GM-11329 from the Office of International Research, NIH, U. S. Public Health Service. t Present address: Hooper Foundation, University of California Medical Center, San Francisco. When movement of the worm is restricted and the position is suitable, ducts and organs in the Mehlis' gland region can be seen. An oil-immersion lens is required for detailed observation of certain parts.