Biomedical Engineering Reference
In-Depth Information
*Trichuris
Arthropoda
Chelicerata (Chapter 29)
Hexapoda (Chapter 30)
Crustacea (Chapter 31)
Platyzoa
Platyhelminthes (Chapter 26)
Acanthocephala (Chapter 28)
Fungi (Chapters 33
37)
Class Bilateria (animals with bilateral symmetry) contains two prominent
subclasses: Class Deuterostomia and Class Protostomia. These two sub-
classes, like their parent class, have bilateral symmetry. Additionally, the
deuterostomes and the protostomes have three germ layers (ectoderm, meso-
derm, and endoderm).
In deuterostomes, a dent forms an anus; the mouth forms later. Humans
and all vertebrates are deuterostomes.
Protostomes also form a dent, early in development, but subsequent
events are somewhat controversial. In most cases, it seems, the primordial
dent in protostomes produces an anus and a mouth.
Three large subclasses of animals comprise Class Protostomia: Class
Platyzoa, Class Ecdysozoa, and Class Lophotrochozoa. Only the first two
classes contain organisms that infect humans. Class Platyzoa contains
Class Platyhelminthes (Chapter 26) and Class Acanthocephala (Chapter 28).
Class Ecdysozoa accounts for Class Arthropoda (Chapters 29
31), and Class
Nematoda (Chapter 27).
Nematodes, also known as roundworms, lack a circulatory system, and a
respiratory system. They are all pseudocoelomates, meaning that their body cav-
ities are not fully lined by mesoderm. Compare this with the flatworms (Class
Platyhelminthes, Chapter 26), that are acoelomate (i.e., without body cavities).
The Nematodes all have a tube-shaped digestive tract, open at both ends (mouth
and anus). The presence of a complete digestive tract is another property that
distinguishes the roundworms from the flatworms; the flatworm digestive tract
has a single opening for the ingestion of food and the excretion of waste.
Nematodes are covered by a cuticle composed largely of extracellular
collagen and other proteins (e.g. cuticulins) excreted by epidermal cells. The
cuticle is shed repeatedly through various life stages of the nematode.
Two major subclasses of Class Nematoda contain the organisms that
infect humans: Secernentea and Enoplea. These two classes have various
anatomic features that distinguish one from the other. Their most relevant
distinction, for healthcare workers, is that members of Class Secernentea are
terrestrial dwellers, while members of Class Enoplea are marine inhabitants.
The many pathogenic members of Class Secernentea are best described
within their subclasses.
