Hong Kong Corals & the Associated Marine Life

> Other marine organisms

Flatworms are most commonly known for their parasitic members including flukes (Class Trematoda) and tapeworms (Class Cestoda). Free-living flatworms (Class Turbellaria) are found in tropical shallow waters. They are the most spectacular members of the group and hence often confused with the nudibranchs.

Most free living flatworms feed on detritus or small invertebrates. They don't have gills for respiration. Oxygen is removed from the surrounding water by diffusion through the skin. It is an inefficient method as it is only effective in small animals. Flatworms normally crawl on the bottom but they can swim with undulations of their thin and flattened bodies.

Phylum Annelida consists of three major groups that include terrestrial (e.g. earthworms), freshwater (e.g. leeches) and marine species (e.g. polychaetes). The distinguishing characteristic of an annelid is the segmented body. 

It is divided into a linear series of similar segments from the head to the tail. Growth involves formation of a new segment anterior to the tail. The head bears sensory organs for processing environmental information.

Most of the polychaetes are marine dwellers. They are extremely abundant and widespread in the ocean. Many of them are benthic (e.g. bristle worms), while some are pelagic and tube-dwelling (e.g. fan-worms). With different body forms, polychaetes feed in different ways, such as deposit feeding, filter feeding and raptorial feeding.