Biomedical Engineering Reference
In-Depth Information
closer relationships to some of the single-celled organisms than those
single-celled organisms had to other organisms in Class Protoctista. Preserving
the concept of Class Protoctista was making it difficult to appreciate the true
phylogenetic relationships among the eukaryotes.
Modern classifications of eukaryotic organisms simply dispense with Class
Protoctista, assigning each individual class of eukaryotes to a hierarchical
position determined by ancestry. A simple schema demonstrates the modern
classification of eukaryotes, and also serves to organize the chapters in Part III
of this topic.
Eukaryota (organisms that have nucleated cells)
Bikonta (2-flagella)
Excavata
Metamonada (Chapter 16)
Discoba
Euglenozoa (Chapter 17)
Percolozoa (Chapter 18)
Archaeplastida, from which Kingom Plantae derives (Chapter 24)
Chromalveolata (Chapters 19
21)
Alveolata
Apicomplexa (Chapter 19)
Ciliophora (ciliates) (Chapter 20)
Heterokontophyta (Chapter 21)
Unikonta (1-flagellum)
Amoebozoa (Chapter 22)
Opisthokonta
Choanozoa (Chapter 23)
Animalia (Chapters 25
32)
Fungi (Chapters 33
37)
Class Animalia (Chapters 25
37) are
much more closely related to Class Choanozoa (Chapter 23), than they are to
Class Plantae (Chapter 24). Likewise, Class Choanozoa is more closely
related to Class Animalia and to Class Fungi than they are to Class
Metamonada (Chapter 16). If we were still using Class Protoctista, we would
have assigned both Class Choanozoa and Class Metamonada to the same super-
class, and we would have blurred the relationships between the subclasses.
Today, the term “protist” or “protoctista” simply refers to any member of Class
Eukaryota that is neither plant, animal, nor fungus.
A quick glance at the eukaryotic schema indicates that the very first division
in the classification of the eukaryotes, is based on the number of flagella: Class
Bikonta (from the Greek “kontos,” meaning pole) consists of all organisms
with two undulipodia; and Class Unikonta consists of all organisms with one
undulipodia [69]. At first blush, the number of undulipodia would appear to be
a poor way of distinguishing classes of organisms; it would seem likely that
32) and Class Fungi (Chapters 33
