Biomedical Engineering Reference
In-Depth Information
Crustacea (Chapter 31)
Platyzoa
Platyhelminthes (Chapter 26)
Acanthocephala (Chapter 28)
Archiacanthocephala
Moniliformida
Moniliformidae
*Moniliformis
Fungi (Chapters 33
37)
Acanthocephalans, also known as thorny-headed worms or spiny-headed
worms, live purely parasitic lives. As is so often the case among dedicated
parasites, these organisms have unburdened themselves of some anatomic
features that may have been useful to their ancestors, but which serve no
purpose when host resources are readily available. For taxonomists, this is a
vexing problem; without inherited anatomic features, it is difficult to estab-
lish an organism's lineage, with any certainty.
Taxonomists have placed the Acanthocephalans as a subclass of Class
Platyzoa, along with the Platyhelminthes (Chapter 26), and Class Rotifera, a
class that does not contain human pathogens. Based on gene comparisons, it
seems likely that Class Acanthocephala will soon be moved a notch over, to
become a subclass of Class Rotifera.
All adult acanthocephalans have an extensible proboscis that is armed
with hooks for attachment to gut wall. There is variation in the size and mor-
phologic features of the acanthocephalans. In the case of the Moniliformis
moniliformis, a known human pathogen, worms, found in stool, are just over
one centimeter in length and about four millimeters wide. The proboscis is
armed with 14 rows, each row with six to eight hooks.
There are well over 1000 species within Class Acanthocephala, infesting
an enormous variety of animals, including dogs, squirrels, rats, birds, fish,
and insects, including cockroaches. Like many dedicated parasitic animals,
members of Class Acanthocephala display a bewildering array of life cycles,
with one or more intermediate hosts. The complete life cycles of many of
the known genera have not been fully established.
Parasitologists have taken a particular interest in Class Acanthocephala
because these parasites have evolved a gruesome strategy for host parasit-
ism known as “brain-jacking.” All studied acanthocephalan species are
able to alter the behavior of their intermediate hosts so as to increase the
likelihood that the acanthocephalan will be delivered to its final host. In
many cases, the parasite provokes the intermediate hosts to move to a
vulnerable location (e.g., the surface of a pond, or a well-lit and exposed
spot of soil), where the final host can eat them. The mechanism underly-
ing brain-jacking is not fully understood, but it seems to be mediated, in
at least some cases, by the parasite-induced release of host serotonin.
