To what phylum does a blood sucking leech belong?

The correct option is Annelida.

Step-by-step explanation:

Leeches (subclass Hirudinea) are segmented parasitic or predatory worms. They are classified under the phylum Annelida due to several key biological characteristics:
1. Metameric Segmentation: Like earthworms, leeches have bodies that are divided externally into ring-like segments (annuli) that correspond to internal compartments.
2. True Coelom: They possess a true coelom (body cavity), which is a characteristic feature of annelids, though in leeches it is modified and filled with botryoidal tissue.
3. Bilateral Symmetry: They exhibit bilateral symmetry and are triploblastic (having three primary germ layers).

Let us look at why the other options are incorrect:
- Aschelmenthis: This phylum contains roundworms (nematodes) which are unsegmented and possess a pseudocoelom (false body cavity).
- Cnidarian: This phylum includes aquatic organisms like jellyfish and hydra, which have radial symmetry and lack segmentation.
- Platyhelminthes: This phylum contains flatworms, which are acoelomate (lacking a body cavity) and mostly unsegmented.