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PACIFIC SCIENCE, Vol. XXII, April 1968 
comprised of leucocytes and/or fibers that in- 
compatible trematode larvae are destroyed. 
Since some indirect evidence suggests that 
substances of host origin, perhaps hormones, 
can alter the normal developmental pattern of 
intramolluscan trematode larvae, it is proposed 
that the absence of growth- and development- 
stimulating substances or the presence of 
growth- and development-inhibiting substances 
may be factors responsible for incompatibility. 
Relative to the nutritional requirements of 
trematode larvae, their availability in the mol- 
lusc, as well as deficiencies in nutriments re- 
sulting from competition between larvae, could 
influence the compatibility of the association. 
Furthermore, the physico-chemical nature of the 
host-parasite interphase could influence the suc- 
cessful or unsuccessful uptake of nutrients and 
hence govern compatibility or incompatibility. 
Finally, in order that the relationship be con- 
sidered a completely compatible one, the para- 
site cannot destroy its host prior to its successful 
escape or the escape of its germ cell-bearing 
progeny. Thus, factors of parasite origin which 
are lethal to the host and factors of host origin 
which prevent escape must also be considered 
as determinants of incompatibility. 
REFERENCES 
Barbosa, F. S. 1965. Ecology of the larval para- 
sitic stages of Schistosoma mansoni. Rev. 
Inst. Med. Trop. Sao Paulo 7:112-120. 
Barbosa, F. S., and M. V. Coelho. 1956. 
Pesquisa de immunodade adquirida homo- 
loga em Australorbis glahratus, nas infesta- 
coes por Schistosoma mansoni. Rev. Brasil. 
Malariol. 8:49-56. 
Barlow, C. H. 1925. The life cycle of the 
human intestinal fluke Fasciolopsis huski 
(Lankester). Am. J. Hyg., Monogr. Ser. 
4:1-98. 
Benex, J., and L. Lamy. 1959. Immobilisation 
des miracidiums de Schistosoma mansoni par 
des extraits de planorbes. Bull. Soc. Pathol. 
Exot. 52:188-193. 
Bils, R. F., and W. E. Martin. 1966. Fine 
structure and development of the trematode 
integument. Trans. Am. Microscop. Soc. 85: 
78-88. 
Brooks, C. P. 1953. A comparative study of 
Schistosoma mansoni in Tropicorbis havenen - 
sis and Australorbis glabratus. J. Parasitol. 
39:159-163. 
Burns, W. C, and I. Pratt. 1953. The life 
cycle of M e tago nimoides oregonensis Price 
(Trematoda: Heterophyidae) . J. Parasitol. 
39:60-67. 
Cable, R. M. 1965. "Thereby hangs a tail." 
J. Parasitol. 51:3-12. 
Campbell, W. C. 1961. Notes on the egg and 
miracidium of Fascioloides magna (Trema- 
toda). Trans. Am. Microscop. Soc. 80:308- 
319. 
Campbell, W. C, and A. C. Todd. 1955. In 
vitro metamorphosis of the miracidium of 
Fascioloides magna (Bassi, 1875) Ward, 
1917. Trans. Am. Microscop. Soc. 74:225- 
228. 
Cheng, T. C. 1961^. Description, life history, 
and developmental pattern of Glypthelmins 
pennsylvaniensis n. sp. (Trematoda: Brachy- 
coeliidae), new parasite of frogs. J. Para- 
sitol. 47:469-477. 
1961 A Studies on the morphogenesis, 
development and germ cell cycle on the spo- 
rocysts and cercariae of Glypthelmins pennsyl- 
vaniensis Cheng, 1961 (Trematoda: Brachy- 
coeliidae). Proc. Penna. Acad. Sci. 35:10-22. 
1962. The effects of parasitism by the 
larvae of Echinoparyphium Dietz (Trema- 
toda: Echinostomatidae) on the structure and 
glycogen deposition in the hepatopancreas of 
Helisoma trivolvis (Say). Am. Zool. 2:513. 
1963^. Biochemical requirements of 
larval trematodes. Ann. N.Y. Acad. Sci. 113: 
289 - 321 . 
1963 A Histological and histochemical 
studies on the effects of parasitism of Muscu- 
lium partumeium (Say) by the larvae of 
Gorgodera amplicava Looss. Proc. Helminth. 
Soc. Wash. 30:101-107. 
1963c. The effects of Echinoparyphium 
larvae on the structure of and glycogen depo- 
sition in the hepatopancreas of Helisoma tri- 
volvis and glycogenesis in the parasite larvae. 
Malacologia 1:291-303. 
1967. Marine Molluscs as Hosts for 
Symbioses: With a Review of Known Para- 
sites of Commercially Important Species. In: 
F. S. Russell, ed., Advances in Marine Biol- 
