4. ANALYSIS OF FACTORS INVOLVED I N SYMBIOSIS
37
behavior than positive phototaxis, and their observations on the
natural dispersal of A. glabratus and 8. mansoni miracidia in vessels
revealed that in the case of both organisms the perimeter of a vessel is
the preferred site. It is of significance to note that Chernin and
Dunavan did state that “ . . . the tropisms (negative geotaxis and
positive phototaxis) do not elicit an absolute response for all miracidia
even when acting together ”.
I n addition to the evidences reviewed above, unfortunately there
have appeared in the literature ‘‘ evidences ” which cannot be critically
analyzed as the result of misunderstandings or misidentifications of the
taxonomy of either the molluscs or the parasites. For example, Cort
(1918)) to illustrate his point that Schistosoma haematobiurn lacks host
specificity and indirectly denying the existence of specific hostelaborated chemotactic agents, stated that in Egypt X. haematobiurn
utilizes both Bulinus contortus and B. dydowski as intermediate hosts
while in South Africa Physopsis africana is implicated. More recent
studies have shown that the two Egyptian snails are synonyms for
Bulinus truncatus. Furthermore, the investigations of McCullough
(1957) and Le Roux (1958) have suggested that the schistosomes
utilizing Bulinus (Physopsis) spp. as intermediate hosts are specifically
distinct from those which utilize B. truncatus. In view of the work of
Newton (1952, 1953, 1954) on Australorbis glabratus infected with
Schistosoma mansoni, it is apparent that successful establishment of
the parasite, which must in most cases be considered distinct from the
initial host-parasite contact, is dependent upon the genetic strain of
the host, and perhaps also of the parasite, beside other factors.
Another example of confusion resulting from misidentification of
molluscan hosts has been cited by Wright (1960) who pointed out that
Stunkard (1957)) in presenting evidence to indicate that a high degree
of host-specificity need not exist in trematode-mollusc relationships,
stated that the intermediate hosts of S. mansoni in Africa are species of
Planorbis, Physopsis and Isidora, while in the West Indies and South
America the snails concerned belong to the genera Australorbis and
Tropicorbis. However, Wright has pointed out that Isadora is a
synonym for Bulinus and Physopsis is a subgenus of Bulinus. He
indicated that the Planorbis referred to by Stunkard is the African
genus Biomphalaria since Planorbis does not occur in the Ethiopian
region. Furthermore, Hubendick (1954) had shown that African
Biomphalaria is congeneric with the New World Australorbis and
Tropicorbis. Thus it would appear that the confirmed natural intermediate hosts of Schistosoma mansoni all belong to one genus. The
fact that these snails are closely related has been demonstrated by
37
behavior than positive phototaxis, and their observations on the
natural dispersal of A. glabratus and 8. mansoni miracidia in vessels
revealed that in the case of both organisms the perimeter of a vessel is
the preferred site. It is of significance to note that Chernin and
Dunavan did state that “ . . . the tropisms (negative geotaxis and
positive phototaxis) do not elicit an absolute response for all miracidia
even when acting together ”.
I n addition to the evidences reviewed above, unfortunately there
have appeared in the literature ‘‘ evidences ” which cannot be critically
analyzed as the result of misunderstandings or misidentifications of the
taxonomy of either the molluscs or the parasites. For example, Cort
(1918)) to illustrate his point that Schistosoma haematobiurn lacks host
specificity and indirectly denying the existence of specific hostelaborated chemotactic agents, stated that in Egypt X. haematobiurn
utilizes both Bulinus contortus and B. dydowski as intermediate hosts
while in South Africa Physopsis africana is implicated. More recent
studies have shown that the two Egyptian snails are synonyms for
Bulinus truncatus. Furthermore, the investigations of McCullough
(1957) and Le Roux (1958) have suggested that the schistosomes
utilizing Bulinus (Physopsis) spp. as intermediate hosts are specifically
distinct from those which utilize B. truncatus. In view of the work of
Newton (1952, 1953, 1954) on Australorbis glabratus infected with
Schistosoma mansoni, it is apparent that successful establishment of
the parasite, which must in most cases be considered distinct from the
initial host-parasite contact, is dependent upon the genetic strain of
the host, and perhaps also of the parasite, beside other factors.
Another example of confusion resulting from misidentification of
molluscan hosts has been cited by Wright (1960) who pointed out that
Stunkard (1957)) in presenting evidence to indicate that a high degree
of host-specificity need not exist in trematode-mollusc relationships,
stated that the intermediate hosts of S. mansoni in Africa are species of
Planorbis, Physopsis and Isidora, while in the West Indies and South
America the snails concerned belong to the genera Australorbis and
Tropicorbis. However, Wright has pointed out that Isadora is a
synonym for Bulinus and Physopsis is a subgenus of Bulinus. He
indicated that the Planorbis referred to by Stunkard is the African
genus Biomphalaria since Planorbis does not occur in the Ethiopian
region. Furthermore, Hubendick (1954) had shown that African
Biomphalaria is congeneric with the New World Australorbis and
Tropicorbis. Thus it would appear that the confirmed natural intermediate hosts of Schistosoma mansoni all belong to one genus. The
fact that these snails are closely related has been demonstrated by