Kingdom: Animalia, Phylum: Nemertinea (Nemertera)
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The Phylum Nemertinea are long, worm-like animals that resemble flatworms in some respects. Their size varies from a few millimeters to several meters. They are almost exclusively marine but some freshwater forms exist and one occurs on damp land. They are all unsegmented. There is no body cavity other than the digestive tract (e.g., no coelom). They are sluggish movers in sand and mud.
They have a large and fancy eversible proboscis, which is not connected to the digestive tract. It is turned inside out food is being gathered, and is used in piercing and entangling prey, which is then brought back to the mouth.
Two classes, Anopla and Enopla are presented. In the Anopla the mouth is located posterior to the brain, the central nervous system lies just below the epidermis and the proboscis is unarmed. In the Enopla the mouth is located anterior to the brain, the central nervous system is inside the body wall muscles and the proboscis is frequently armed.
The Digestive Tract is a straight tube and this is the first animal that possesses an anus.
Respiration is by diffusion and there are no specialized structures. Excretion is accomplished with flame cells.
The Circulatory System consists of closed blood vessels. There is a dorsal blood vessel and two lateral branches of it. No heart is present and the movement of blood is accomplished in two ways: (1) by contraction of the walls of the vessels and (2) by movements of the animals themselves. There are red corpuscles in the vessels, which is unusual because most invertebrates have haemoglobin dissolved in the plasma. The haemoglobin distributes oxygen and may also distribute food and wastes.
The Body Wall is similar to that of Turbellaria. A ciliated epidermis occurs that is supplied with numerous gland cells. There are well-developed circular and longitudinal muscles in the mesenchyme.
Reproduction is both sexual and asexual. In sexual reproduction there are mostly separate sexes. The gonads open to the surfaace by ducts, and fertilization is external in the water. The following diagram shows a typical life cycle:
Asexual reproduction is by fragmentation. Here the body breaks up into many pieces, which are capable of regenerating to a new worm. This not only serves reproduction but is also for protection of the animal as a whole.
Importance.-- There is one parasite on the egg masses of the blue crab, but otherwise they are not important economically. However, from an academic point of view these are the lowest animals with a complete digestive tract with a mouth and anus and they possess a circulatory system with blood, corpuscles and haemoglobin. They also have a separate proboscis.
Please see following plate for Example Structures of the Nemertinea:
Plate 63 = Phylum: Nemertinea -- Example Structures