Polycladida

One of the diverse groups which has wide distribution in the coral reefs worldwide. The polyclad worms associated with the coral reefs are not yet explored from Andaman and Nicobar Islands.  Preservation of organism is almost impossible for this group, and may be the reason for lack of studies by the earlier workers who were depending on the specimens most of the work. Digitalization makes a possible scope for the detailed study of this group from the coral reefs. Acanthozoon species was observed many a times from the reefs of South Andaman region. More or less 10 species were observed during the surveys of ZSI in the whole archipelago. 

The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals. Unlike other bilaterians, they have no body cavity, and no specialized circulatory and respiratory organs, which restrict them to flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion.

Because they do not have internal body cavities, for over a century Platyhelminthes were regarded as a primitive stage in the evolution of bilaterians (animals with bilateral symmetry and hence with distinct front and rear ends). However, analyses since the mid-1980s have separated out one sub-group, the Acoelomorpha, as basal bilaterians, in other words closer to the original bilaterians than to any other modern groups. The remaining Platyhelminthes form a monophyletic group, in other words one that contains all and only descendants of a common ancestor that is itself a member of the group. The redefined Platyhelminthes is part of the Lophotrochozoa, one of the three main groups of more complex bilaterians (Source: Wikipedia)