Which of the following animal phyla is commonly known as the flatworms because its members have flattened, ribbon like or leaf like bodies?

Introduction / Context:
In zoology, different animal groups are placed into phyla based on key structural and developmental features. Many phyla have common names that reflect a characteristic body plan. Flatworms are one such group, named for their dorsoventrally flattened bodies. This question checks whether you can associate the common name flatworms with the correct formal phylum name from among several major animal groups.

Given Data / Assumptions:

• The common name flatworms is given, referring to animals with flattened bodies.

• Several phyla are listed: Mollusca, Chordata, Ctenophora, Platyhelminthes, and Arthropoda.

• We assume basic familiarity with the key examples and body types found in each phylum.

• The task is to select the phylum whose members are collectively known as flatworms.

Concept / Approach:
The phylum Platyhelminthes includes animals such as planarians, flukes, and tapeworms. The name comes from Greek roots platy meaning flat and helminth meaning worm. These animals are acoelomate, bilaterally symmetrical, and have thin, flattened bodies that facilitate diffusion of gases and nutrients in the absence of specialised respiratory and circulatory systems. Mollusca includes snails and squids, Chordata includes vertebrates like fish and humans, Ctenophora includes comb jellies, and Arthropoda includes insects and crustaceans. None of these other groups are called flatworms.

Step-by-Step Solution:
Step 1: Recall that flatworms are simple, soft bodied animals lacking a true body cavity and having flattened bodies. Step 2: Recognise that examples such as tapeworms and liver flukes, which are medically important parasites, belong to this group. Step 3: Connect the name Platyhelminthes with flatworms by remembering that platy means flat and helminth means worm. Step 4: Consider Mollusca, which includes animals such as snails, mussels, and squids that usually have shells or muscular bodies, not flat ribbon like forms. Step 5: Chordata includes animals with a notochord at some stage, such as fish, birds, and mammals, which are not flatworms. Step 6: Ctenophora contains comb jellies, gelatinous marine organisms, and Arthropoda contains joint legged animals like insects and crabs, none of which fit the description of flatworms. Step 7: Therefore, the correct phylum that is commonly called flatworms is Platyhelminthes.

Verification / Alternative check:
Zoology textbooks and classification charts consistently list flatworms under Phylum Platyhelminthes. The fact that medical parasitology sections describing tapeworms and flukes refer to platyhelminth infections confirms the link between this phylum name and the flatworm body form. Major phylum overviews also describe Platyhelminthes as dorsoventrally flattened, unsegmented worms, reinforcing that this is the correct association.

Why Other Options Are Wrong:

Mollusca: Includes snails, bivalves, and cephalopods, many of which have shells or muscular bodies, not flattened worm like forms.

Chordata: Contains vertebrates and some invertebrates with a notochord, such as humans and fishes, not flatworms.

Ctenophora: Marine comb jellies with radial symmetry and gelatinous bodies, very different from flatworms.

Arthropoda: Joint legged animals with exoskeletons, including insects and crustaceans, not simple flattened worms.

Common Pitfalls:
Students sometimes confuse different worm groups such as flatworms, roundworms, and segmented worms. Remember that Platyhelminthes are flatworms, Nematoda are roundworms, and Annelida are segmented worms. Keeping these three major worm phyla distinct, along with their names and body plans, reduces confusion during classification questions.

Final Answer:
The animal phylum commonly known as flatworms is Platyhelminthes.