88 Bulletin American Museum of Natural History [Vol. LIIL 
the discovery that certain water-snails act as intermediate hosts of 
helminth diseases. Recent researches seem to indicate that most, if not 
all, trematode parasites of vertebrates spend part of their life-cycle. in 
some mollusk or crustacean. Although we are here primarily concerned 
with the rdle of fresh-water mollusks in the transmission of parasitic 
worms, it may not be amiss to mention that many terrestrial snails and 
slugs, as also some marine mollusks, may act as intermediate hosts to 
certain of these parasites. A well-known example is that of Leucochlori- 
dium paradoxum Carus, ‘a frequent parasite in the tentacles of Succinea 
putris (Linneeus), in Europe, which upon entering song- and water-birds 
develops into the trematode parasite of the cloaca known as “ Distomum 
macrostomum” Rudolphi.! Grassi and Rovelli? have shown that one of 
the tapeworms of chickens, Davainea proglotiina (Davaine), in Europe 
has as intermediate hosts the slugs Limax cinereus Lister, L. agrestis 
Linnaeus, and L. variegatus Draparnaud. 
All flukes, or parasitic trematode worms, pass in the course of their 
ontogeny through a number of distinct phases, accompanied by migra- 
tions to different hosts and intervening free-living stages. The develop- 
ment of the common liver fluke, Fasciola hepatica Linneus, a cosmopoli- 
tan parasite of sheep, goats, and other ruminants, and accidentally of 
man, may be taken as a classic illustration. In this case the adult para- 
site lives in the bile passages and liver tissue of the vertebrate host; its 
eges are carried by the bile to the intestine and thence out of the body 
with the feces. The eggs hatch upon reaching water at a favorable 
temperature, producing minute, ciliated embryos known as ‘“miracidia,” 
which swim about in search of a suitable intermediate host. In Europe 
this is the common pond snail, Lymnea truncatula (Miller), as discovered 
by Leuckart in 1882.23 Upon meeting the snail, the miracidium bores, 
into its body by means of a small, papillary projection of the anterior end 
and transforms within the tissues of the mollusk into a sporocyst. This 
produces a large number of parthenogenetic eggs, each developing into a 
larva of a second type known as a “redia.”’ The wall of the sporocyst 
eventually bursts and the reclis invade the tissues of the snail to develop, 
at least under certain conditions, into a new generation of rediz. The 
second, or sometimes a third, generation of redize produce by partheno- 
1Zeller, E. 1874. ‘Ueber Leucochloridium paradoxum Carus und die weitere Entwickel sel 
on bruh el Veena Zool., XXIV, pp. 564-578, 1 Pl. ha taal 
Grassi, B. an ovelli, G. 1889. ‘Embryologische Forschungen an Cest ty 
Bakt. u. Parasitenk., V, pp. 370-377, 401-410. ese ee ee entra. f. 
. See Leuckart, R. 1886-1901. ‘Die Parasiten des Menschen,’ 2d Ed., I, Abt. 2, pp. 258-264. The 
ee id of Ae ce, ee ee ee also worked out independently by A. P. Thomas in 
2 ee omas, A. P. . ‘The life-history of the liver-fluke (Fasciola’ h ca).’ 
Journ. Microsc. Sci., N.8., X XIII, pp. 99-133, Pls. oe oo oo 
¢ 
