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Phylum Platyhelminthes, commonly known as flatworms, exhibit the following key characteristics:

• They are bilaterally symmetrical and triploblastic, meaning they are composed of three distinct cell layers.
• Flatworms lack a true body cavity other than their gut, and they do not possess an anus. Food intake and waste expulsion occur through the same pharyngeal opening.
• In larger flatworms, the gut is often highly branched to facilitate the transport of food to all parts of the body.
• Due to the absence of a body cavity, flatworms rely on diffusion for respiration. This physiological constraint necessitates their characteristic flattened shape, ensuring no cell is too far from the external environment for gas exchange.

The diagram illustrates the internal and external morphology of a flatworm (Planarian). The important morphological features are identified below:

1. Eyespots (Ocelli): Light-sensitive organs located on the anterior (head) region.
2. Auricles: Ear-like projections on the sides of the head, containing chemoreceptors for sensing chemicals in the water.
3. Epidermis/Body Wall: The outer protective layer of the organism.
4. Gastrovascular Cavity: A highly branched digestive system that distributes nutrients throughout the body.
5. Pharynx: A muscular, eversible tube located mid-ventrally, used for feeding.
6. Nerve Cords: Longitudinal nerve bundles forming part of the ladder-like nervous system.
7. Protonephridia/Excretory Canals: Part of the excretory system, involved in osmoregulation and waste removal.