Biology Reference
In-Depth Information
ultrastructure, life cycles, and embryonic development of the group, to
which the reader is referred. Illustrations for the discussion presented here
can therefore be kept to a minimum.
The Aspidogastrea ( ¼ Aspidobothrea ¼ Aspidobothria) is a taxon
belonging to the phylum Platyhelminthes. Within the phylum, it is the
sister group to the digenean trematodes, as shown unambiguously by
morphology, ultrastructure, and molecular data. It contains species parasitic
in molluscs and vertebrates. For some species, molluscs serve as intermediate
hosts harboring larvae and juveniles, for others, they may also act as
definitive hosts in which maturation of the worms occurs. Vertebrates
become infected by eating infected molluscs, at least in the species for
which the life cycle is known. Aspidogastreans are likely to be very ancient,
originating probably several hundred million years ago (Figure 10.1 ) . There
is controversy about whether molluscs or vertebrates are the original hosts.
Three of the four families, each with a single genus and one or two
species, infect chondrichthyan fishes (sharks, rays, and chimaeras), species
of the fourth family occur in teleost fishes and turtles. Chondrichthyans
are over 400 million years old, and the few digenean trematodes infecting
them have almost certainly been acquired secondarily from teleost fishes.
On the other hand, the relatively great diversity of aspidogastreans in
chondrichthyans at least at the family and genus level suggests that these
fish are their original hosts.
Aspidogastreans are organisms of remarkable complexity. Whereas
almost all digenean trematodes possess one or two suckers, the aspidogas-
treans have a row of ventral suckers or a ventral disc that is subdivided
into many suckerlets. The nervous system of the only species, Multicotyle
purvisi, examined in detail to date has more longitudinal and transverse
nerves than any other platyhelminth, and there is an extraordinary
variety of sensory receptors. Aspidogastreans appear to be only ''super-
ficially'' adapted to a parasitic way of life. Whereas digenean trematodes
can be kept alive outside a host only in artifical media of great complex-
ity, simulating the environment of the host and the chemical com-
pounds provided by it, aspidogastreans have been kept alive outside a
host for many days or even weeks in water or saline solution. Also, host
specificity among aspidogastreans is very low, that is, a species typically
infects many molluscan and vertebrate hosts, or - if a species is specific -
specificity seems to be due to ecological and not physiological factors.
However, many species have not been examined in this way.
In the following, I discuss two species in detail, Multicotyle purvisi and
Lobatostoma manteri (Rohde 1968 , 1972 , further references therein, 1973 ,
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