Order Stichosomida
Rev.
08/31/21
- esophagus
with multiple glands,
- vertebrate and invertebrate parasites
- Nematodes formerly classified in the Order Stichosomida constitute at
least two important groups of animal parasites, the
mermithids, parasites of invertebrates, and the
trichinellids,
parasites of vertebrates. These nematodes are characterized by a pharynx
that is narrow and thin-walled anteriorly and which, posteriorly, is
surrounded by unicellular, glandular stichocytes, each with a duct into the
pharyngeal lumen.
- The pharynx extends one-fourth to nine-tenths of the body length
in various taxa and is almost devoid of musculature.
- The region of the pharynx surrounded by stichocytes is known as the
stichosome.
- Recent phylogenetic analysis based on a synthesis of molecular and
morphological data suggest that the stichosome may be an example of parallel
evolution and that the trichinellids and mermithids are more appropriately
separated as two orders:
Trichinellida, with at least six families, and
Mermithida, with two families (De
Ley and Blaxter, 2002).
- However, debate on the higher classification of nematodes continues and
opinion will no doubt refocus as new data emerge and are applied (Ferris,
2007).
- Two brief life history examples provide some insight into the biology of
these organisms.In Trichinella spiralis, encysted larvae are
ingested in infected muscle tissue (raw or
undercooked pork is the
classic example in the case of trichinosis in humans). The cyst
surrounding the larva is digested in the new host and the larvae molt to
adults, mate,
embed in the intestinal epithelium and females produce eggs
which hatch (1000 larvae per female in 5 days). The hatched larvae are
distributed via the circulatory system and
migrate into surrounding
cells, which die unless they are striated muscle fibers. Secretory
products of the stichocytes induce DNA endoreduplication and transformation
of the
muscle fiber and into a multinucleate nurse cell which becomes
encapsulated by collagen and supplied with capillaries (Despommier, 1998;
Lee, 2002). The life cycle continues
when the muscle is eaten by another
animal. In Romanomermis culicivorax, the preparasitic larva is equipped
with a stylet which allows penetration through the cuticle
of the
mosquito larva host. The stylet is lost in subsequent larval stages and the
stichosome separates from the intestine, which becomes a trophosome with no
anal
opening to the exterior. Nutrient uptake from the insect hemolymph
is through a very thin cuticle, perhaps enhanced by narrow diameter
pores. Interestingly, the epidermal
cells adjacent to the cuticle have
their absorptive surface area increased buy outwardly directed microvilli
(Poinar and Hess, 1977; Munn and Munn, 2002). The continued role
of the
stichosome is unclear although it appears to be involved in protein
synthesis (Poinar, 1983). The larva undergoes one molt in the insect host
and then ruptures the
body wall as it emerges. After a final molt to the
adult stage, the female deposits eggs. The nature and function of
stichocyte secretions, particularly of vertebrate parasites, is an
area
of continued interest in research on host-parasite relationships.
References:
Ferris, H. 2007.
Stichosomida. McGraw-Hill Encyclopedia of Science & Technology.
http://accesscience.com/abstract.aspx?id=757377&referURL=http%3a%2f%2faccesscience.com%2fcontent.aspx%3fid%3d757377.
Go to Nemaplex Main Menu