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
Cuticular modifications at the anterior ends of many
nematodes may include leaf crowns, vesicles, alae and papillae.
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
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 papillae are cuticular protuberances believed to be sensory in
function. They vary in shape from small "button-like" protuberances to longer
Caudal alae are wing-like expansions of the cuticle and may be found
at the tail ends of nematodes.
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.