Polyclad Flatworms (Phylum: Platyhelminthes, Class: Turbellaria, Order: Polycladida), although not related to molluscs (Phylum: Mollusca) they are often mistaken for sea slugs (Order: Nudibranchia) because of their brilliant colour patterns.
The genus Prostheceraeus has only two sub categories: Prostheceraeus giesbrechtii (photo to the left) and Prostheceraeus roseus (photo to the right). They belong to the family Euryleptidae, order Polycladida and class Turbellaria.
Polycladida is a highly diverse group of free living flatworms. Members of this order are characterized anatomically by having a highly branched and irregular gut and a pharynx plicatus (a highly ruffled pharyngeal tube).
The Platyhelminthes (Greek: platys-flat, helminthes-worm) which belong to the kingdom Animalia are unsegmented flat worms with a head and a tail end. They are considered the most primitive bilaterally symmetrical animals. Bilateral symmetry means that their body exists in mirror images about a long anterior-posterior axis with definite upper and lower surfaces and anterior and posterior ends.
Furthermore, flatworms are triploblastic, which means that body structure is based on three fundamental cell layers (endoderm, mesoderm and ectoderm).
As a third characteristic, they have no body cavity (coelom) other than the gut, an organization which is called acoelomate. Respiratory system and blood vessel system are also completely missing and therefore, diffusion is used for transport of oxygen inside the body. This constraints flatworms to be flat as for maintaining metabolism, no cell can be too far from the outside, making a flattened body shape necessary.
Almost all species are hermaphrodites with a quite complex reproductive system. The majority of turbellarian flatworms are marine and can be found in the oceans where they are almost all benthic and most common in shallow water.
Most common, polyclads are active carnivorous predators and scavengers and can be found feeding on various sessile invertebrates. Some species are herbivores and have specialized on green algae or benthic diatoms.
Some free living flatworms show a fascinating ability of regeneration. Cut off its head and it grows a new one. Divide its head laterally into two, three or more parts and a two, three or multi-headed worm will result. Worms can be cut into ten parts and ten complete smaller worms grow, one from each fragment!
Both the above photos were taken at Reqqa Point on Gozo’s north coast.
Photos taken by Brian Azzopardi