Looking at the animal kingdom, it is obvious that
there is a great deal of diversity in body form. These differences in
form are called morphological characteristics and are often used to
separate animals into their respective taxonomic groups.
The web sites linked from here are designed to
focus on the major defining characteristics of animals and how they
are used to divide the kingdom by means of Morphological
diversity as well as:
Invertebrates and the Origin of Animal Diversity
- Animal life began with the Precambrian
evolution of multicellular marine forms that lived by eating other
- Subsequent evolution has generated a diversity
of animals, grouped into about 35 phyla. About 95% of animal
species are invertebrates
- Animals are multicellular eukaryotes
distinguished be a specific type of heterotrophy called
- Animal cells lack walls and store carbohydrate
reserves as glycogen. In most animals, cells are successively
organized into tissues, organs, and organ systems.
- Animal reproduction is primarily sexual, and
the life cycle is dominated by the diploid stage, gametes
generally being the only haploid cells -- Which life cycle does
this represent --sporic, gametic, or zygotic. Asexual budding or
regeneration occurs in some species. Fragmentation also
- In sexual reproduction, fertilization of an
egg by a flagellated sperm initiates cleavage in the zygote and
the formation of a hollow ball of cells called the
blastula. The next stage in development is a infolding of
the blastula similar to shoving your fist into a beach-ball called
a gastrula (see below).
- Many animals go through metamorphosis, a
second stage of development that transforms a sexually immature
larva into a morphologically distinct sexual adult.
- Muscles and nerves, which control active
behavior, are unique to animals.
Clues to Animal Phylogeny.
- Taxonomists rely mainly on comparative anatomy
and embryology to reconstruct animal phylogeny.
- Eumetazoa (all animals except sponges)
diverged early into two major branches, the Radiata (radial
symmetry) and the Bilateria (bilateral symmetry). Members of the
branch Radiata are the jellyfish and their relatives, sedentary
(polyp) and planktonic (medusa) forms with radial symmetry.
Members of the branch Bilateria are characterized by bilateral
symmetry and cephalization (a head with cluster of sensory
- Bilateral animals develop from embryos
constructed of three concentric primary germ(inal) layers: an
inner endoderm, an outer ectoderm, and a middle
- The body plan of Bilatria is either solid, as
in the acoelomates, or has a digestive tube separated from the
outer body wall by a cavity (the coelom). In pseudocoelomates, the
cavity is incompletely lined by embryonic mesoderm. Coelomates
have a true coelom, a body cavity lined by mesoderm and attached
on either end by mesentery tissue.
- Based on features of embryonic development,
coelomate phyla are divided into two main groups: the protostomes,
comprising the annelids, mollusks, and arthropods; and the
deuterostomes, consisting of the lophophorate animals,
echinoderms, and chordates.
Parazoa (Phylum Porifera)
- The least complex animals are the sponges,
which lack tissues, such as muscles and most organs. The body is a
sac perforated with holes
- Sponges filter-feed by drawing water through
pores into a central cavity, the spongocoel, and out an osculum.
Lining the inside of the sponge body are choanocytes with are
flagellated cells with a membranous collar around the base of the
- Phylum Cnidaria (nydaria) consists of
primarily marine carnivores possessing tentacles armed with
stinging cnidocytes (each containing stinging capsules called
nematocysts) that aid in defense and the capture of prey. The
simple, radial body exists as a sessile polyp or a floating medusa
(or both) organized around a central opening which serves as both
a mouth and anus.
- Phylum Ctenophora, the comb jellies, are
transparent animals whose locomotion depends on eight rows of
- Phylum Platyhelminthes, the flatworms, are the
simplest members of Bilatria. Ribbonlike animals with a single
opening to their gastrovascular cavities. Representatives of the 4
classes include planaria, free-living heterotrophs, flukes,
parasites in animals, and tapeworms (Cestoda) parasites with a
headlike scolex connected to a ribbon of detachable units of sex
organs called proglottids.
- The proboscis worms of the phylum Nemertea are
named for the retractable tube they use for defense and prey
capture. This group has a simple circulatory system and a complete
digestive tract with both mouth an anus.
- Phylum Rotifera is made up of tiny animals
possessing a crown of cilia that draws food into the
- The roundworms of the phylum Nematoda are
among the most numerous animals in both species and individuals.
Nematodes have tapered ends. The phylum includes both free-living
species and parasites, including the nematode that causes