Return to
Table of

Return to
Biology 2108
Dr. D.'s
KSU Home
Biology 2108 Lecture
Biodiversity: Animals

Defining characteristics of animals

Theories origin of animals

Syncytial theory - Metazoa evolved from multinucleated ciliate
('syncytial' refers to protoplasm that contains numerous nuclei not separated from each other by plasma membranes).

Colonial Theory
-Metazoa evolved from hollow, spherical colonial flagellates

Flagellate protozoans formed a colonial hollow sphere of organisms, termed a blastaea then increasing division of labour including invagination to form a jellyfish-like gut. 

Recent  molecular data supports a monophyletic origin of metazoans, and that choanoflagellate protists are sister to the monophyletic Metazoa. Does this suggest that two or more of these theories might be correct concerning the origin of metazoans?


Early history of multicellular animals

Cambrian Explosion - the rapid (~10 million years) appearance of major animal taxa in the early Cambrian.
  Several important fossils discoveries have extended the length of time over which this diversification occurred.

Cambrian macrofauna explosion
-First fully skeletonized organisms, many that are clearly related to modern groups, as well as other taxa that can not be confidently placed into modern phyla

The Shelly Fauna
- The first evidence of animals with hard parts are fossils that are small (generally 1 to 5 mm).  
The earliest ‘trace’ fossils - Burrows and intense disturbance of sediment layers (clear indications of trace fossils as early as 570 million years ago) indicate the presence of soft-bodied animal with relatively sophisticated muscular and nervous systems.
Ediacara “fauna”  - Quilt-like body plans forming feathery fronds, pouches, disk and worm-shapes, without heads or obvious circulatory, nervous or digestive systems.  There is controversy over whether these organisms are ancestors of modern animals, a failed experiments’ in early animal evolution that left no modern day representatives,  or not even multicellular animals at all.

The bottom line is that the evolution of multicellular animal life may be older than previously believed but was still relatively rapid in geological terms (40 million years), and when compared with life today produced most of the major animal body plans.  The last 500 million years has been mostly tinkering.

Modern major groups of animals
There are ~35 modern phyla of animals.  However, the evolutionary relationships among these groups have been difficult to determine, in part due to their relatively rapid apparence during the cambrian explosion.
Below is a older ('traditional) version of the animal phylogenetic tree (don't memorize this one):

parazoa e.g. sponges
radiata e.g. jelly fish

acoelomate e.g. flatworms 


pseudocoelomate  e.g. rotifers, roundworms

protostomes  e.g. annelids, molluscs, arthropods



deuterostomes  e.g starfish, chordates 

There has always been some controversy concerning this tree of the major groups of animals.  Characteristics used in traditional animal groupings have been questioned (e.g. some of these characteristics may be the result of neotony, or comparisons are made from shared primitive characteristics).

More recently, researchers using molecular analysis (DNA/RNA sequencing) have proposed  major changes in the animal phylogenetic tree (a simplified version below), forcing a re-examination of morphological data:


ANIMALS platyzoa




e.g. sponges 
e.g. jelly fish   
e.g. flatworms. rotifers  
e.g. annelids, molluscs   
e.g. arthropods, round worms  

 e.g starfish, chordates  

Major phyla
Characteristics Examples


two "layers",
typiclly amorphous (no symmetry)

Phylum Silicea

porous body plan with spicules,
often of silica


Phylum Calcarea

porous body plan with spicules
of calcium carbonate

Why is the former Phylum Porifera
now split into two

more sponges

two tissue layers,
radial symmetry

Phylum Ctenophora
Eight "comb rows" of cilia
for locamotion,

sticky cells called colloblasts


Tentacles with cnidoblasts

jellyfish, corals


phyla in this group are related to
one another (and to the
based mainly on molecular similarities

Phylum Platyhelminthes
flatten veriform body
without body cavity
flat worms including
tapeworms and flukes


Phylum Rotifera
Small (<3mm), typically transparent,
often with corona and trophi

phlya in this group either share
a similar larval type (trochophore)
or a similar feeding structure

Phylum Mollusca
head, foot, and fleshy mantle clams, snails, octopi

Phylum Annelida
segmented veriform body earthworms, polychaetes,  and leeches

Phylum Brachiopoda
lophophore within pair of calcified shells


Phylum Bryozoa (Ectoprocta)
colonial form, each individual
with a lophophore

phlya in this group all possess
a cuticle (non-cellular out covering)
that molts

Phylum Arthropoda
jointed exoskeleton of
crustaceans, insects, arachnids, etc.

Phylum Nematoda
round veriform body, cuticle,
muscles only longitudinal
hookworms, pinworms, and other

phlya in this group share development
that is characterized by
a number of
developmental characteristics

the mouth not derived from the

Phylum Echinodermata
pentamerous radial symmetry,
water vascular system,
larvae similar to hemichordates
sea stars, urchins, sea cucumbers, etc.

Phylum Hemichordata pharyngeal gill silts,
dorsal, hollow nerve cord
larvae similar to echinoderms
acorn worms

Phylum Chordata
pharyngeal gill silts
dorsal, hollow nerve cord
tunicates, fish, reptiles, birds, mammals


Derived characteristics in chordates that led to mammals:

Return to
Table of

Return to
Biology 2108
Dr. D.'s
KSU Home