>ERIC WETZEL wrote:
>>>> Are folks comfortable with the idea that cercariae that
>> encyst "on" something are in general ancestral to those that encyst
>> "in" a next intermediate host? Is there a nice, neat evolutionary
>> trend from encysting on a plant to encysting on an animal to
>> encysting 'in' an animal? Any discussion (to the group or to me) is
>> welcome.
> Hrmmm, well, just to play Devil's advocate, why could it not be that
>cercariae that encyst on something were derived from cercariae that
>encyst in another host? Thus, it would seem that there are 2
>possibilities:
>1) cercariae that encyst on something gave rise to cercariae that need
>another host
>or
>2) cercariae that encyst in another host gave rise to those that encyst
>on something.
(there is more)
__________________________
>Mark Rigby
Also to play Devil's advocate:
It seems to me that the signals to start encystment *in* a host must be more
foreign to a cercaria than the signals *on* a host. After all, the environment
*on* a host is largely the same as for the free cercaria. As a biological
event, I would expect that responding to the signals *in* a host is a more
complicated physiological procedure than responding to the signals *on*
a host. Therefore, I would expect that the *ecto* encystment is ancestral
to the *endo* encystment.
Greetings,
Omar O. Barriga
