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Nematodes resemble annelids and flatworms, but are more robust and elongate than most flatworms, and lack annelids' segmentation. In a few species, the epidermis is annulated, and internal organs such as gonads and nephridia make serials. On the other hand muscles, nerves and other internal structures do not form serials.
The body wall is composed of a dense ciliated, a layer of connective tissue and a thick musculature. It has no cuticle, and each ciliated cell has many cilia and microvilli.
There are 1150 nematodes species, or ribbon worms.
Nematodes, like arthropods and tardigrades, lack motile cilia.
Most species are less than 20 cm long, some are only a few millimeters long, but the genera Cerebrat and Lineus may be a meter long. In some species, nematodes' long, slender are longer than 1 m. One from St. Andrews, Scotland, was a 54 m boot-lace worm, the longest animal on Earth.
The smallest nematodes are circular or only slightly flattened in cross section, where species which are larger-bodied are flattened and ribbon-like.
While some species are pale and nondescript, many, including some that live in darkness, have patterns and pigments of yellow, orange, red and green.
The central nervous consists of a brain and paired long nerve cords.
Nematodes lasso or harpoon their requimes with a sticks, penetrating or venomous proboscis.
Feeding and Excreting
Like flatworms, nematodes transport oxygen across the body wall.
Many burrow in sediments, in crevices or the roots of algae and sessile animals, and some speices make gelatinous lairs in deep water. A few species live as ectosymbionts in the mantles of bivalves, in the atrium of tunicates or on crabs.
About 12 species live in fresh water, and about 15 primarily live in humid tropics and subtropics. 
Reproduction and Development
Nemerteans readily regenerate, and reproduce both clonally and sexually.
Diverstity of nemerteans
Bottom-feeding fish, some shore-birds, and other invertebrates such as horseshoe crabs, and also other species of nemerteans eat other nemerteans. Nemerteans' edidermis secretes a sticky, toxic mucus to discourage predators, and nemerteans' bright and contrasting colours advertise their bad taste.
The North American "Cerebratulus lacteus" and the South African "Polybrachiorhynchus dayi" are sold as fish bait. These species are not related to true tapeworms, and are not parasites.
Most of the characters shared by nemerteans and flatworms are ... 
- Ruppert, E.E (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. ISBN 0030259827. Unknown parameter
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