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PACIFIC SCIENCE, Vol. XXII, April 1968 
Reaching a suitable site 
(absence of barriers) 
Overcoming host's defense mechanisms 
(innate and acquired cellular reactions; 
innate and acquired (?) humoral reactions 
Host elaborated stimulating 
and inhibiting substances 
(hormones?) 
Nutrition 
Non- lethal to host 
Fig. 5. Diagrammatic drawing illustrating factors which can govern or influence the successful establish- 
ment of trematode larvae in a mollusc. 
hosts’ alimentary tracts rather than between the 
hepatopancreatic lobules— as is the case with 
certain other plagiorchioid trematodes (Mattes, 
1936; Maldonado, 1945; Cort et ah, 1952) — 
is that the migration of these sporocysts be- 
comes restricted by their hosts’ basement mem- 
branes. Although the mother sporocysts of both 
H. intestinalis and G. quieta become successfully 
established at this site and produce daughter 
sporocysts, the migration of the mother sporo- 
cysts does become restricted. Schell (1962^) 
has stated: "In following the development of G. 
quieta it became evident that the thin basement 
membrane beneath the intestinal epithelium 
of the host snail plays an important role in pro- 
tecting the snail from invasion by the parasite.” 
Is it possible, then, that certain species of trema- 
todes, which could not undergo normal develop- 
ment at the site restricted by the basement mem- 
brane, would in this way be prevented from 
becoming established? Although examples of 
this, as far as I have been able to determine, 
have not yet been described among mollusc- 
larval trematode relationships, the condition 
known as schistosome dermatitis caused by avian 
schistosome cercariae in human skin is an ex- 
ample of the inability of these carcariae to suc- 
cessfully penetrate the depth of the abnormal 
host’s skin: being unable to survive in the 
surfacial areas of the skin, they die. In this con- 
nection, Lewert and Lee (1954), Lewert 
(1958), and Lewert and Mandlowitz (1963) 
have demonstrated that the physical and chem- 
ical natures of the basement membrane and 
ground substance do determine whether the 
entrance of invasive forms of helminths will be 
successful. 
The evidence presented indicates that bar- 
riers, especially in the form of basement mem- 
brane and perhaps ground substances, situated 
in the path of invading trematode larvae could 
prevent them from reaching a satisfactory site 
for further development and thus serve as 
determinants of host incompatibilty. 
Host’s Defense Mechanisms 
What is known about the nature of internal 
defense mechanisms in molluscs, both cellular 
and humoral, has been reviewed in recent years 
by Stauber (1961), Cheng and Sanders (1962), 
Tripp (1963), and Cheng (1967). The role of 
molluscan leucocytes (amoebocytes or phago- 
cytes) and fibrous elements in innate immunity 
by causing the encapsulation of trematode lar- 
vae, is well established. The results of Newton 
(1952, 1954), Brooks (1953), Sudds (I960), 
and others strongly indicate that encapsulation 
generally occurs around larvae in unnatural 
hosts. However, slight and restricted encapsula- j 
tion may also occur around larvae in their 
natural hosts (Cheng and Cooperman, 1964; 
Probert and Erasmus, 1965; Schell, 1961, 
1962^, h; Pan, 1965), but these extremely light 
capsules usually inflict little or no damage upon 
