Nematoda Phylum (Nemata): Wire-shaped worms

The nematodes or nematodes (Nemathelminthes), also called cylindrical worms, are considered the most abundant group of metazoans in the biosphere, estimated to constitute up to 80% of all metazoans with more than 20,000 species already described, out of an estimated over 1 million current species, which include many parasitic forms of plants and animals.

Only Arthropoda are more diverse. The name comes from the Greek word. nema, which means thread.

The nematodes successfully conquered the marine, freshwater and terrestrial habitats. Although most are free-living, there are many parasitic representatives of virtually all types of plants and animals. Its size is very variable, ranging from about 1 mm to about eight meters in length.


Are animals triblastics (have the 3 germinal leaflets: ectoderm, mesoderm and endoderm), pseudocellomates (body cavity is bounded by mesoderm tissues and endoderm tissues), protostomies (when the blastopore gives rise to the mouth) and have bilateral symmetry.


The body of these worms is covered by a very resistant protective cuticle produced by the epidermis, composed mainly of collagen. This cuticle protects against enzymes produced by the digestive system of the host organism. The epidermis is composed of a layer of simple cells.


The musculature of the nematodes is composed of a single layer of cells that is distributed longitudinally throughout the body. This smooth musculature is responsible for the movements of these animals. They cause dorsoventral flexions. Movement will also depend on the elasticity of the cuticle and the hydrostatic skeleton, the liquid present in the pseudoceloma.


Nematodes have no respiratory system, and breathing is cutaneous or cut off, made through diffusion.


Nematodes are the first animals to present complete digestive system, that is, they have mouth and anus.
The mouth has lips around it. These lips have sensory papillae, teeth, or cutting plates followed by a muscular pharynx that sucks food pumping into the intestine. Parasites feed on pre-digested products by the host, but there are also phytophagous and carnivorous species.


They have no circulatory system. The circulation of gases, nutrients and toxic substances is made by the pseudoceloma.


They have a specialized cell, shaped like the letter H. They have two longitudinal channels, which run along the side of the worm's body, joined by a transverse channel, which emits a duct that excretes through the excretory pore. The main excreta of these animals is ammonia.

Nervous system

Have two nerve cords that run through the animal's body, ventrally or longitudinally. From the pharynx depart the nerve cords. The dorsal nerve cord is responsible for motor function, while the ventral nerve is sensory and motor, being considered the most important.


Are animals dioeciousMost of them have separate sexes. They present sexual dimorphism. That is, the female is different from the male. Usually males are smaller and their posterior portion is tapered and curved to facilitate copulation. Fertilization is crossed and development is indirect.

Differentiation between male (curled end) and female (straight end) of Ascaris

Diseases Caused by Nematodes




Hookworm: Yellowish