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PACIFIC SCIENCE, Vol. XXII, April 1968 
attraction in governing host-specificity in 
mollusc-trematode relationships. This topic, 
which has been critically reviewed by myself 
(Cheng, 1967), among others, is still a con- 
troversial one. The controversy is not whether 
host-attraction does occur since, in my opinion, 
the studies of Faust and Meleney (1924), Faust 
(1934), Faust and Hoffman (1934), Barlow 
(1925), Tubangui and Pasco (1933), Mathias 
(1925), Kloetzel (1958, I960), Kawashima 
et al. (1961a), Campbell (1961), Davenport 
et al. (1962), Etges and Decker (1963), and 
Maclnnis (1965) have demonstrated rather 
conclusively that attraction between miracidia 
and molluscs does occur. This is a subtle phe- 
nomenon, however, which is operative only 
within very restricted distances and can be ob- 
served only with the application of quantitative 
techniques. The controversy is over the question 
whether chemotaxis is in any way related to 
host-specificity and hence influences compatibil- 
ity. Although the studies of Faust and Meleney 
(1924), Barlow (1925), Neuhaus (1953), and 
Etges and Decker (1963) suggest that miracid- 
ial attraction is host-specific, the results of Sudds 
(I960), Kawashima et al. (1961a), and Bar- 
bosa (1965) suggest that attraction of miracidia 
to a specific mollusc need not be correlated with 
subsequent compatibility. 
Experiments carried out in our laboratory 
have revealed that the miracidia of Fasciola 
gigantica are stimulated by the plasma and tissue 
extracts of laboratory-raised Galba ollula, the 
natural host in Hawaii. These reactions, how- 
ever, are not specifically elicited by G. ollula, 
since similar reactions were observed when 
miracidia were exposed to the plasma and tissue 
extracts of two other species of freshwater 
gastropods, Tarebia granifera mauiensis and 
Helisoma duryi normale. 
In the first series of experiments, F. gigantica 
miracidia, between 15 and 25 minutes post- 
hatching, were placed in shallow Petri dishes 
(50 mm in diameter, 15 mm high) in which 
had been placed specific dilutions of the plasma 
or tissue extracts of H. duryi iiormale, T. grani- 
fera mauiensis, or G. ollula. Each dish was 
placed over a grid marked off in 2.54-mm 
squares. Using a y i0 -second-interval stop-watch 
and observing under a dissection microscope 
illuminated by indirect lighting, the swimming 
speed of a single miracidium was timed, re- 
corded as seconds/2.54 mm, and later expressed 
as mm/sec (Tables 1 and 2). As controls, the 
swimming velocities of miracidia of similar age 
placed in distilled water were determined. 
The six test media consisted of 1:10, 1:50, 
and 1:100 dilutions of molluscan plasma and 
similar dilutions of tissue extracts. Blood was 
collected from the molluscs’ body sinuses by 
gently cracking the shell of each snail, without 
injuring the soft tissues, and permitting the 
blood to drain to the lower edge of an inclined 
Stender dish from whence it was rapidly col- 
lected with a hypodermic needle and syringe. 
The cellular components of whole-blood samples 
were removed by centrifugation. The tissue ex- 
tracts were prepared by homogenizing the soft 
tissues of each snail in 1 cc of distilled water 
after the tissues had been completely desan- 
guinated and perfused with running distilled 
water for 15 minutes. After homogenization in 
an ice bath, the homogenates were centrifuged 
and the aqueous extracts collected were con- 
sidered the "concentrated” extracts. 
All snails used were laboratory-raised and 
known to be parasite-free. The concentrated 
plasma and tissue extracts of each species were 
pooled and the desired dilutions were made 
from the pooled samples. All observations were 
made at 22 ± 1° C. 
From the data presented in Tables 1 and 2, 
it is evident that all three dilutions of the 
plasma and tissue extracts of G. ollula (the I 
natural host), H. duryi normale, and T. grani- 
fera stimulated F. gigantica miracidia to increase | 
their swimming velocities. In addition, the mira- 
cidial swimming pattern was conspicuously 
altered. Miracidia in distilled water usually i 
swam linearly, rotating along their longitudinal ! 
axes. Divergences from such a course were grad- : 
ual rather than abrupt. When placed in plasma 
or extracts, their swimming behavior became 
erratic. They turned abruptly and frequently. 
In the second series of experiments, 10 mira- 
cidia, 10-15 minutes post-hatching, were placed 
in small Petri dishes (60 mm in diameter, 13 
mm high) which contained 10 cc of distilled 
water. In the center of each dish was placed an 
agar block of three mm 3 which had been pre- 
soaked in concentrated plasma or tissue extracts 
of G. ollula, H. duryi normale, or T. granifera j 
