One example of animals having a single opening to the outside that serves both as mouth as well as anus is:
Octopus
Asterias
Ascidia
Fasciola
Phylum Platyhelminthes (Flat Worms) -
Free living, commensal or parasite with dorsoventrally flattened body, bilaterally symmetrical, triploblastic and acoelomate.
- wherein
Eg. Taenia, Fasciola
Fasclola (Platyhelminthea) has single opening for both ingestion and excretion
Option 1)
Octopus
This is incorrect option
Option 2)
Asterias
This is incorrect option
Option 3)
Ascidia
This is incorrect option
Option 4)
Fasciola
This is correct option