The Phylum Annelida is defined by its morphology as segmented worms. They express closed circulatory systems in the same fundamental manner that Homo sapiens do. Annelids are categorized taxonomically into many classes. A few of these classes are: Class Polychaeta, Class Clitellate and Class Echiuran. Common organisms in the Class Clitellate include Oligochaeta (earthworms) and Branchiobdellida ad Hirudinea (leeches). Ecologically, Annelids are subdivided into two classifications. Errent polychaetes express a multitude of parapodia appendages, defined heads complete with proboscis and are able to extend their bodies in order to have a greater physical surface area for gas exchange. The latter group being Sessile Tube Dwelling Annelids. These organisms contain the segmented part of their bodies within an external tube. The portion of their being that is extended is a feeding appendage that filters through the water. A great example of a Sessile Tube Dwelling Annelid is the Christmas-tree worm. Each Christmas-tree worm has two feeding appendages to filter through the water. They also have the ability to retract quickly when the worm feels threatened. Some are even photosensitive, which means when natural light is obscured by a potential predator, they automatically retreat. Sessile annelids are also inclusive of medusa worms and deep-sea thermal vents.