Biomedical Engineering Reference
In-Depth Information
bodies can be divided into two symmetrical halves by a plane that runs along
a central axis). Class Bilateria includes all mammals and most large, com-
plex animals. With one exception, members of Class Bilateria have a body
cavity, known as a coelum. The exception is Class Platyhelminthes, the flat-
worms (from Greek “platys”, flat, “helmis”, worm). The prevalence of body
cavities in all other members of Class Bilateria suggests body cavities are
important. Humans have specialized body cavities in which organs are sus-
pended. For example, the lungs are suspended in the pleural cavity, and the
heart is suspended in the pericardial cavity. Lacking these cavities, members
of Class Platyhelminthes lack lungs and a cardiovascular system. Oxygen is
absorbed by simple diffusion; hence, platyhelminths are flat to maximize
oxygen transfer.
Before going further, it is important to understand some of the taxonomic
confusion that arises whenever the term “worm” is used to describe a human
infection. Parasitic worms are called helminths. Class Platyhelminthes
(Chapter 26) and Class Nematoda (Chapter 27) account for all the so-called
helminthic diseases of humans.
Readers should be warned that the term “worm” has no taxonomic mean-
ing; soft, squiggly organisms colloquially known as “worms” are scattered
throughout animal taxonomy, with no close relationship to one another.
A small squirming organism referred to as a “worm” may be an insect larva
(i.e., not a helminth), or it may be one of several unrelated classes of organ-
isms. Class Acanthocephala (Chapter 28) includes the thorny-headed worms.
Class Annelida (earthworms) descends from Class Lophotrochozoa, which
includes molluscs. Class Nematoda (roundworms) and Class Annelida (earth-
worms) are more closely related to spiders and clams, respectively, than
either one is related to Class Platyhelminthes (flatworms). Many so-called
worms are actually the larval forms of animals whose adult stage bears no
resemblance to worms. An example is Linguatula serrata (Class Crustacea,
Chapter 31), the agent causing tongue worm disease. The tongue worm is
the larval stage of a crustacean. Likewise, the screw-worm (Cochliomyia
hominivorax) is actually a type of fly. It is called a screw-worm because the
disease is manifested by worm-like larvae growing in skin. The ineptly
named “ringworm” infections are not caused by worms or by any animals;
they are fungal infections of the skin. The word “worm” may even refer to a
marsupial joey (Class Mammalia), which is typically a smooth hairless slug-
shaped organism the size of a jellybean. When a word cannot be applied to a
set of objects that are mutually related, what meaning does it convey? When
you come across one of these forms of life, you either know its proper taxo-
nomic class, or you do not. If you cannot classify the organism correctly,
then no generic term will suffice.
Perhaps the most striking property of Class Platyhelminthes is its remark-
able developmental plasticity. Class Turbellaria, a subclass of platyhelminths
that contains no human pathogens, is adept at growing whole organisms
