Physical characteristics

The majority of adenophoreans are free-living, microbot-rophic, and aquatic nematodes. Only a few species are plant parasitic, invertebrate parasites, or vertebrate parasites. They range in size from microscopic to as long as 3.25 ft(1 m) in exceptional cases. Adenophoreans are considered non-segmented pseudocoelomates; that is, creatures possessing a three-tissue-layered body that has a fluid-filled body cavity (pseudocoelom) between the endoderm and the mesoderm (the innermost and middle tissue layers).

A flexible but durable collagenous cuticle protects the body with a series of grooves across the body from head to tail. The non-cellular cuticle, which generally has a smooth surface, can sometimes contain transverse or longitudinal striations and has four layers: endocuticle, epicuticle, exocuticle, and meso-cuticle. The cellular hypodermis is the subcuticular layer that secretes the cuticle. Phasmids—which are minute pore-like chemoreceptors that (when present) are usually paired—are generally absent from adenophoreans. Their sensory system contains well-developed amphid apertures, which are postlabial (past the lips) in position, with some species having apertures that are labial (on the lips). The apertures are variable

Roundworm Anatomy
Roundworm anatomy. (Illustration by Christina St. Clair)

in external shape, being sometimes circular, pocketlike, porelike, or spiral.

Somatic and cephalic setae, which are elongated structures joined with the cuticle, are common. These tactile sensory organs are usually located around the oral openings. The cephalic sensory organs, which number 16, are setiform to papilloid, and post-labial or labial in position about the head. Deirids, which are paired, porelike organs located in the lateral fields near the nerve ring, are present in some species.

Usually present, hypodermal glands are a thin tissue layer beneath the cuticle that thickens to make the dorsal, lateral, and ventral chords and extend the body's length. In general, they are tactile sensory organs usually located around the oral openings. They contain uninucleate hypodermal cells. A layer of longitudinal muscles underlies the hypodermis.

Bursae (or caudal alae) are rarely found. The ventrally located excretory system, when present, is usually single-celled, usually with non-cuticularized terminal ducts, and lacking collecting tubules. A rectal gland is usually absent. When present, there are three caudal glands located near the posterior region. The muscular esophagus or pharynx (the tube that moves food from the mouth/head to the stomach/intestine) varies in configuration, but the majority of adenophoreans have three esophageal glands: two that are subventral and one that is dorsal. The subventral glands open into the posterior metacarpus. The dorsal gland opens anteriorly into the procorpus or the anterior metacarpus. Its basic structure is corpus (the anterior part is cylindrical) with the basal (bottom) region sometimes swollen in the shape of a bulb. The glands empty their contents into the esophagus to aid in digestion. The tail is the region between the anus and the posterior tip. The male tail is smooth, and lateral cuticular caudal extensions of the tail rarely occur.

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