Quiz 14: Acoelomorpha, Platyzoa, and Mesozoa
Bilateral symmetry is a symmetrical arrangement of an organism along the central axis, such that the organism can be divided into two equal halves. Hence, this is also called plane symmetry. This symmetry is a characteristic feature of animals, which can move freely in their environment. This symmetry is very essential for them to seek food, shelter, home, and reproductive mates. Active directed movement requires an elongated body form with both head and tail. This is possible with bilateral symmetry and so it has become an adaptive value for actively motile animals.
The appropriate terms which match with the terms in the other column is: • Turbellaria - Free-living and commensal • Monogenea - Ectoparasites • Trematoda - Endoparasites • Cestoda - Endoparasites
Turbellarians have mouth on the ventral side which leads into gut cavity, often via a pharynx. Turbellarians are often distinguished on the basis of the form of gut and pharynx. Except for order Polycladida, Turbellarians with endolecithal eggs have a simple gut and a simple pharynx. In a few Turbellarians there is no recognizable pharynx. Polyclads have a folded pharynx and a gut with many branches. Polyclads have a folded pharynx and a gut with many branches.