The marine environment includes a far greater diversity of animal groups than the terrestrial environment, which is not surprising since living organisms first appeared in the seas several hundred millions years before life on land evolved. Whether in the sea or on land, most plant and animal species are grouped into assemblages or communities characteristic of recognisable habitats.

The annelids, also known as the ringed worms or segmented worms, are a large group, with over 22,000 species including ragworms, earthworms, and leeches. The basic annelid form consists of multiple segments. Each segment has the same sets of organs and, in most polychates, has a pair of parapodia that many species use for locomotion.

An arthropod is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans.

Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick.

The adults are recognizable by their (usually five-point) radial symmetry, and include starfish, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies or “stone lilies”.

Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. They are highly diverse, not just in size and anatomical structure, but also in behaviour and habitat. The three most universal features defining molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system.

Platyhelminthes are relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates (having no body cavity), and have no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion.

Based on what you know now, are you gonna be able to guess from which groups are those marine organisms?

Look at the diversity of marine life! So many different shapes and colors!