Anatomy
Nemerteans are distinguished by there having a long protrusible proboscis that is housed in a separate fluid filled cavity called the rhynchocoele, this proboscis may be coiled within the rhynchocoele and be longer than the animals body. The rhynchocoele is generally as long as the body and leads anteriorly to a short passage called the rhynchodaeum which open via the proboscis pore at the front end of the body. In most species the proboscis pore is completely separate from the animals mouth and digestive tract which opens some way back on the body. In some species however the proboscis and the digestive tract share a common opening. The proboscis itself can be either armed with spines or unarmed.
Nemerteans have a muscular body wall that includes both circular and longitudinal muscles. These muscles may be arranged so that there is one layer of each with the circular muscles outside the longitudinal ones, or there may be two layers of circular muscles sandwiching a layer of longitudinal muscle or there may be two layers of longitudinal muscle sandwiching a single layer of circular muscle.
Nemerteans have a well developed blood system with two types of blood vessels but no heart, the blood vessels have valves and some have muscle layers that allow them to constrict thus causing the blood to flow. The blood of Nemerteans is general colourless, though is some species the corpuscles may be coloured green or red.
Nemerteans have a well developed nervous system with cephalic ganglia and a nerve network connecting various sensory organs. Usually there are a pair of cerebral ganglia, a pair of large ganglionated nerve cords as well as main nerve net. Many Nemerteans possess eyes, from two to two hundred per individual, they are located towards the front of the animals body near the cephalic ganglia. Other sense organs possessed by Nemerteans include cephalic grooves and sensory pits both with chemotatic functions.
The Nemertean gut is basically a ciliated tube running from the mouth to the anus, a metabolic excretory system exists in most Nemerteans in the form of pairs of nephridial ducts, there may be from two to three hundred pairs of ducts in a single animal.