The Nematodes


External Structures - cuticle

Nematodes are covered by a protective outer skin called a cuticle which is naturally colorless and partly translucent. The cuticle also lines the buccal cavity, esophagus, excretory pore, vagina, cloaca and rectum.

The cuticle is also resistant to host digestive enzymes and in most nematodes is relatively impervious, allowing only the passage of water molecules and certain small water- soluble ions.

The cuticle also appears to function as part  of what Noble et al describe as the "hydrostatic skeleton of nematodes".   Since the body cavities of nematodes contain pressurized fluids the cuticle apparently serves to maintain the body at a constant diameter by resisting the internal pressure of these fluids. In keeping this fluid contained, the cuticle therefore maintains a nematode's form and structure and also provides an anchor for muscles. The patterns of the several cuticular layers and their contents appear to allow simultaneous radial strength and longitudinal flexibility. In other words, the cuticle layers are arranged so as to maintain a constant body diameter while, at the same time, allowing the nematode to stretch longitudinally.

A variety of organic compounds have been identified in the cuticles of many nematodes. These include amino acids, proteins, carbohydrates, lipids, RNA, ascorbic acid, ATP and hemoglobin. Their presence and variety suggests that the cuticle is far from inert and is, in fact, metabolically activity most of the time.

It is also known that the cuticle is antigenic and may play an important role in eliciting the immune responses of infected hosts.

In many nematodes the cuticle is a smooth outer layer but in others the cuticle may have longitudinal and circular striations and may be  modified to produce a variety of structures that can be useful in identification of specific nematodes.


Cuticular modifications at the anterior ends of many nematodes may include leaf crowns, vesicles, alae and papillae.

smantenddraw.JPG (43743 bytes)Leaf crowns (A) are rows of finger-like projections surrounding the rim of the opening to the buccal cavity. They are particularly obvious in the strongyles of horses.

Vesicles are inflations of the cuticle around the mouth - cephalic (B) and the anterior esophagus -cervical (C).

Alae are, as the name suggests, "wing-like" expansions of the cuticle. Cervical alae (D) are located in the terminal half of the esophageal region when cervical vesicles are also present and cover most of the esophageal region in the absence of vesicles.

Cervical papillae (E) are paired spine-like projections found in the esophageal region. Their function is believed to be tactile or sensory.


Click here to see a set of images showing specific nematodes with cuticle modifications of their anterior ends.

Cuticle modifications - posterior end


Cuticle modifications at the posterior ends of nematodes may include caudal papillae and caudal alae.

Caudal papillae are cuticular protuberances believed to be sensory in function. They vary in shape from small "button-like" protuberances to longer "stalk-like" structures.

Caudal alae are wing-like expansions of the cuticle and may be found at the tail ends of nematodes.

Smpostdraw.JPG (39264 bytes)In male nematodes, of species belonging to the order Strongylida, caudal alae are greatly expanded to form a structure called a copulatory bursa - so called because it is used by the male to grasp the female during copulation.

A bursa has two lateral lobes and in some species, a third dorsal lobe. The bursa is supported by finger-like structures called rays which are caudal papillae with associated muscle tissue.

Each lateral lobe usually contains six rays and the dorsal lobe has one ray.The number and shape of these rays may be valuable characteristics for species identification of nematodes in the order Strongylida.

When resting, the bursa looks like a relaxed, folded hand but during copulation it is greatly expanded and used to grasp the female.


Click here to see a set of images of specific nematodes showing modifications to the cuticle at their posterior ends.

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Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000