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One example of animals having a single opening to the outside that serves both as mouth as well as anus is
A
$Fasciola$
B
$Octopus$
C
$Asterias$
D
$Ascidia$
Detailed Solution
A digestive system with only a single opening to the outside, which serves as both mouth and anus, is called an incomplete digestive system (blind-sac plan).
It is found in coelenterates and in platyhelminthes.
$Fasciola$ (liver fluke) is a flatworm of phylum Platyhelminthes, so it has an incomplete alimentary canal with a single opening.
$Octopus$ (Mollusca), $Asterias$ (Echinodermata) and $Ascidia$ (Urochordata) have a complete digestive system with separate mouth and anus.
Hence the example is $Fasciola$.
It is found in coelenterates and in platyhelminthes.
$Fasciola$ (liver fluke) is a flatworm of phylum Platyhelminthes, so it has an incomplete alimentary canal with a single opening.
$Octopus$ (Mollusca), $Asterias$ (Echinodermata) and $Ascidia$ (Urochordata) have a complete digestive system with separate mouth and anus.
Hence the example is $Fasciola$.