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PACIFIC SCIENCE, VoL XXII, April 1968 
mauiensis for 10 hours. Although exact be- 
havioral patterns exhibited by miracidia in the 
proximity of an agar block were determined, as 
well as the number of miracidia in each con- 
centric zone at various time intervals, these 
results will not be reported at this time. Only 
the total number of contacts made by the mira- 
cidia per 5 minutes during the initial 10-minute 
period are reported (Table 3). The same num- 
ber of miracidia placed in distilled water with 
untreated agar blocks served as controls. All 
observations were made at 22 dz 1° C. 
From the data presented in Table 3, it is 
apparent that significantly more contacts were 
made between miracidia and plasma- and tissue 
extract-soaked agar blocks than with control 
blocks. The exception was in the case of T. 
granifera tissue extract-soaked blocks. 
It may be concluded then that the miracidia 
of F. gigantica respond to a velocity-increasing 
stimulant and an attractant in the plasma and 
tissue extracts of certain species of molluscs, 
and that these factors are not host-specific and 
need not indicate successful subsequent pene- 
tration and development. It is noted that the 
velocity-increasing stimulant and the attractant 
may be the same factor. 
As has been pointed out in an earlier review 
(Cheng, 1967), the effectiveness of the "host 
factor” (a term introduced by Davenport 
(1955) to designate the stimulatory material of 
host origin) in guiding miracidia to their mol- 
luscan host is doubtful under certain circum- 
stances. For example, Etges and Decker (1963) 
have pointed out that the naturally-occurring 
negative geotaxis and postive phototaxis of 
Schistosoma mansoni miracidia most probably 
eclipse the chemotactic effect of the "host fac- 
tor.” Even between these taxes, Chernin and 
Dunavan (1962) have demonstrated that the 
negative geotaxis is a more powerful determi- 
nant of miracidial behavior than is positive 
phototaxis. Thus, it is only under those condi- 
tions where naturally-occurring taxes guide the 
miracidia to the immediate proximity of the 
mollusc that the influence of the "host factor,” 
which is operative only within short distances, is 
effective (Fig. 1). 
Fig. 1. Diagrammatic drawing illustrating factors which can govern or influence miracidium-mollusc con- 
tact during the pre-invasion phase. The terms "tropism” and "taxis” are used interchangeably. 
