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Publisher Summary A comparison of helminth life-cycles reveals a number of singular features in the digenean pattern. Thus, in addition to the most singular feature, alternation of generations, there is the ubiquity of the cercaria, a stage designed for escape from the molluscan first host and for swimming, two patterns of behavior that are not called for in many life-cycles. To explain the ubiquity of the cercaria, it is postulated that the present molluscan first intermediate host was the original host of the proto-digenean and that escape of the cercaria from this host is primitive. And to explain the occurrence in many life-cycles of a free-swimming miracidium that penetrates the integument of the molluscan host, it is postulated that in an earlier stage the proto-digenean was an ectoparasite of the mollusc. Assuming that the proto-digenean was a visceral parasite of a mollusc and that it escaped from its host as an adult, presumably tailed, to lay its eggs, then the known life-cycles of contemporary digeneans may be interpreted as suggesting the following order in the acquisition of hosts: (i) vertebrate definitive (two-host life-cycle) and (ii) invertebrate second intermediate (three-host life-cycle). With regard to the succession of generations, it is postulated that the present adult was the original adult, that the mother sporocyst generation was the first of the new generations, that the redia was the second, and that daughter sporocysts have been derived from rediae several times.

Galactosomum timondavidi, sp. n., from Larus argentatus Pont. taken near Marseille, France, differs distinctly from all other species in the genus in the following combination of characters: (i) spines on the wall of the ventrogenital sac, (ii) small, symmetric, unarmed ventral sucker, (iii) bilobed gonotyl, and (iv) undivided seminal vesicle. Of these, the most singular feature is the presence of spines on the wall of the ventrogenital sac. The single specimen identified as Microlistrum (= Galactosomum) cochleariforme by Joyeux and Baer (1928), and reidentified as Knipowitschetrema nicolai by Dubois and Mahon (1955), proved on examination to belong to G. timondavidi. Among material collected from gulls by one of us (G.P.) were nine specimens of Galactosomum. Study of these revealed that they represent a new species, and are identical with the single adult recorded by Joyeux and Baer (1928) as Microlistrum (= Galactosomum) cochleariforme from Macedonia, and with the metacercaria reported as Knipowitschetrema echinatum by Timon-David (1970). The species is named in honor of the late Prof. J. Timon-David of Marseille, who contributed greatly to the study of heterophyid and other trematodes in the Mediterranean. Galactosomum timondavidi sp. n. (Figs. 1-3)