Compatibility in Trernatodes and Molluscs — Cheng 
153 
From the study cited above, it would appear 
that some host factor (s), perhaps hormones, in 
O. silicula with healthy gonads does disrupt the 
normal development of M. oregonensis rediae 
and metacercariae during the later phase of the 
relationship. The significance of this finding to 
our discussion is that it is an example of a host- 
elaborated substance which "inhibits” normal 
development and thus promotes incompatibility. 
The phenomenon reported by Meade and Pratt, 
however, does not appear to hold true in all 
transplanted mollusc-larval trematode associa- 
tions. Chernin (1966), for example, has re- 
ported successful transplants of S. mansoni 
mother and daughter sporocysts from B. glabrata 
to acceptor snails of the same species which 
were followed by normal cercarial formation. 
Perhaps this difference can be explained by the 
fact that M. oregonensis includes a redial stage 
while S. mansoni includes sporocyst stages. It is 
well known that rediae inflict significantly more 
damage upon molluscs than do sporocysts. Thus, 
perhaps the donor B. glabrata is never com- 
pletely castrated and S. mansoni, as the result of 
a long relationship with B. glabrata and ex- 
posure to its hormones, is not adversely affected 
by hormones during the latter phases of its de- 
velopment, as is M. oregonensis. 
Another example of possible host-stimulated 
developmental alterations among larval trema- 
todes has been reported by James (1964) and 
discussed by Cable (1965). James reported that 
the intramolluscan life cycle stages of the 
gymnophallid trematode Parvatrema homoeotec- 
num include a "primary germinal sac” with 
adult features, including an oral sucker, ventral 
sucker, pharynx, and bifid caeca, and a "daugh- 
ter germinal sac” which is unique in that, in 
addition to the adult features found on the 
"primary germinal sac,” it also possesses a 
bifurcate tail. The "daughter germinal sacs” 
increase in size and lose their tails while still 
within the "primary germinal sac.” Further 
development does not occur until they rupture 
out of primary sacs. Daughter sacs then continue 
to develop in one of three possible ways: (1) 
most produce cercariae and metacercariae; (2) 
a few produce a second generation of "daughter 
germinal sacs”; and (3) very occasionally, cer- 
cariae, metacercariae, and second-generation 
"daughter germinal sacs” are produced in the 
same "daughter germinal sac.” These larval 
stages are found in the haemocoelic spaces of 
the hepatopancreas and gonad of Littorina saxa- 
tilis tenebrosa. According to James, the "primary 
germinal sac” could be interpreted to be a 
metacercaria ; while according to Cable, the 
"daughter germinal sac” could be considered a 
cercaria. Thus, the usual sequence of larval 
stages is reversed in P. homoeotecnum. Since the 
usual life history pattern among related gym- 
nophallids includes two molluscan intermediate 
hosts, both being marine pelecypods (see 
Stunkard and Uzmann, 1958), Cable has given 
the following as one possible explanation for 
this variation: "It may be significant that 
metacercariae of other gymnophalline species 
live in loose, even superficial association with 
their hosts whereas the species that James 
(1964) described invades the snail to the extent 
commonly seen in molluscs serving as the first 
intermediate host of trernatodes in general. 
As a result, that species probably gets a double 
exposure of the most intimate sort to the tissues 
and body fluid of mollusks.” This, of course, 
implies that the tissues and body fluids of 
molluscs may have influenced that unusual de- 
velopmental sequence. Although in this instance 
the presumably host-stimulated developmental 
alterations do not affect the parasite deleteri- 
ously, it is conceivable that such changes could 
in certain instances deter or inhibit delayed 
polyembryony and thus render the host in- 
compatible. 
Nutrient Requirements 
Available evidence indicates that trematode 
parasites utilize carbohydrates as their primary 
energy source. They acquire their carbohydrates 
in the form of glucose resulting from the degra- 
dation of the host’s stored glycogen or, if there 
is no stored glycogen in the vicinity of their 
natural habitat, they utilize the mollusc’s blood 
sugars (Cheng, 1963^). In addition to sugars, 
these larvae apparently utilize free amino acids 
from the mollusc’s hemolymph and perhaps 
even from the surrounding host cells which 
are lysed or ruptured mechanically. 
Lipids, in the form of short-chain fatty acids, 
are also taken up by germinal sacs, but are 
primarily stored rather than utilized in cer- 
cariae and in certain species in the walls of 
