**Nematoida**
**Definition**
Nematoida is a clade of ecdysozoan animals that includes the phyla Nematoda (roundworms) and Nematomorpha (horsehair worms). These organisms are characterized by their elongated, cylindrical bodies and a cuticle that they periodically shed.
**Overview**
Nematoida comprises two closely related groups of worms that share morphological and developmental traits. Members of this clade are primarily pseudocoelomates, possessing a body cavity not fully lined with mesoderm. They exhibit bilateral symmetry and undergo ecdysis, the molting of their external cuticle.
**Phyla within Nematoida**
– **Nematoda:** Also known as roundworms, nematodes are highly diverse and abundant, inhabiting marine, freshwater, and terrestrial environments. Many species are free-living, while others are parasitic in plants, animals, and humans.
– **Nematomorpha:** Commonly called horsehair worms, these are mostly parasitic during their larval stage, infecting arthropods, and free-living as adults in aquatic habitats.
**Significance**
Nematoida species play important ecological roles, including nutrient cycling and population control of host species. Some nematodes are significant in agriculture and medicine due to their parasitic nature.
**META_DESCRIPTION**
Nematoida is a clade of ecdysozoan worms including nematodes and horsehair worms, characterized by their cylindrical bodies and molting cuticle. They occupy diverse habitats and have ecological and medical importance.