STUDIES ON DIGENETIC TREMATODES OF THE GENERA GYMNOPHALLUS AND PARVATREMA

Abstract

The genus Gymnophallus was erected by Odhner (1900) to contain Distowum deliciosum Olsson, 1893 and other small species from the gall bladder, intestine and bursa Fabricii of shore-birds. Subsequent observations have shown that the asexual generations of these worms occur in bivalve mollusks and that the cercariae, which are produced in sporocysts, belong to the Dichotoma group of furcocercous larvae. Typically, these cercariae have eye-spots and short bifid tails, although either or both may be reduced or absent. Cercaria dichotowza emerges and swims as a furcocercous larva. I n certain species the tail undergoes regression and is lost before the larva emerges from the sporocyst whereas, in others, apparently no tail is formed. On emergence from the first intermediate host, the cercariae attach to the mantle or body wall of bivalve or gastropod mollusks where as unencysted metacercariae, they develop to almost definitive size. The metacercariae may produce lesions on the mantles of their hosts and such injuries stimulate proliferation of tissues, especially of the secreting layer of the mantle, and deposition of nacreous material. Despite the observations of many investigators over a period of more than fifty years, no complete life-history has yet been worked out and the specific relations between particular cercariae, metacercariae, and sexually mature worms remain undetermined. The presence of pearly formations in the mantle of Mytilus edulis has been known for at least three hundred years. According to Giard (1907), they were reported by Olaus Worm in 1655 from mussels taken at Roeskild, near Copen-hagen. Robert Garner observed them in M. edulis from the English coast and he (1872) recognized that they were formed as a reaction by the mollusk to a small distome parasite on the mantle. Baron d'Hamonville (1894) found pearls (sans valeur) in M. edulis at Billiers (Morbihan) France, although the infection was limited to the area of the port. Giard (1897) reported small distomes, often associated with irregularly shaped calcareous deposits, between the mantle and shell of Donax trunculus L., Tellinu fabula Gronov, Tellina tenuis DaCosta and Tellina solidula (= T. balthica L., e x parte) from Boulogne-sur-Mer. The worms were 0.5 mm. long, with rudiments of testes, but no ovary, and Giard suspected that they might be stages in the life-cycle of Brachycoelium luteum (van Beneden), a parasite of the common dogfish, Scyllium canicula. The larger specimens were often less active, more opaque, and filled with sporozoans (GlugidCes). Dubois (1901) studied the parasites of M. edulis at Billiers and found them in reddish brown spots,