Further Studies on the Development of the Sporocyst Stages of Plagiorchiid Trematodes

Abstract

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 snail's 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 snail's 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 snail's tissue, grow thicker and appear crowded with cercarial embryos. Soon, thickened areas of the true sporocyst wall appear at their ends, which frequently