There are nine hundred species of nemerteans. They are elongate, flattened worms. They are found in marine mud and sand. They have a long proboscis. Therefore, nemerteans are commonly called proboscis worms. Adult worms range in size from a few millimeters to several centimeters in length. Most nemerteans are pale yellow, orange, green or red. Its characteristics are:
- They are triploblastic. acoelomate. bilaterally symmetrical unsegmented worms.
- They possess a ciliated epidermis containing mucous glands
- They have complete digestive tract with an anus
- Excretory structures are protonephridia.
- IlJervous system is composed of cerebral ganglion, longitudinal nerve cords, and transverse commissures.
- They have closed circulatory system.
- Body musculature forms two or three layers.
Proboscis: The nernerteans have long proboscis. It is the most distinctive feature of this group This proboscis is held in a sheath called a rhynchocoel. The proboscis has a barb callvii called a stylet. Carnivorous Species use the proboscis to capture annelid and crustacean prey.
Digestive system: The nemerteans have a complete one-way digestive tract. They have a mo ‘all for ingestion of food and an anus for eliminating digestive wastes. The food moves from anterior to posterior part of digestive cavity and mechanical breakdown of food, digestion, absorption, and feces formation take place. It is a great innovation in nemerteans.
Circuulatory system: They also have nervous system. It is another major innovation. Its circulatory system consists of two lateral blood vessels. These lateral blood vessels have branched tributary. Heart is absent in them. The contractions of the walls of the large vessels propel blood. Blood does not circulate. It simply moves forward and backward through the longitudinal vessels. Blood cells are present in some species.
Evolutionary advancement in Nemertea
The nemerteans have two advance characters. Therefore, they are much larger than the most flat worms.
(a) The presence of blood vessels. These blood vessels provide blood to local tissues.
(b) They have one-way digestive system. It can process nutrient efficiency.
Reproductive system: Nemerteans are dioecious. Male and female reproductive structures develop from parenchyma.
Locomotion: The adult nemerteans glide on a trail of mucus Cilia and peristaltic contractions of body muscles provide the propulsive forces.
Fig: Phylum Gastrotricha. The internal anatomy of a Fresh gastrotrticha.