Animal Evolution


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


  1. Animal life began with the Precambrian evolution of multicellular marine forms that lived by eating other organisms.
  2. Subsequent evolution has generated a diversity of animals, grouped into about 35 phyla. About 95% of animal species are invertebrates


Defining "Animals"

  1. Animals are multicellular eukaryotes distinguished be a specific type of heterotrophy called ingestion.
  2. Animal cells lack walls and store carbohydrate reserves as glycogen. In most animals, cells are successively organized into tissues, organs, and organ systems.
  3. 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 occurs.
  4. 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).


  1. Many animals go through metamorphosis, a second stage of development that transforms a sexually immature larva into a morphologically distinct sexual adult.
  2. Muscles and nerves, which control active behavior, are unique to animals.

Clues to Animal Phylogeny.

  1. Taxonomists rely mainly on comparative anatomy and embryology to reconstruct animal phylogeny.
  2. 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 organs)
  3. Bilateral animals develop from embryos constructed of three concentric primary germ(inal) layers: an inner endoderm, an outer ectoderm, and a middle mesoderm.
  4. 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.
  5. 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)


  1. The least complex animals are the sponges, which lack tissues, such as muscles and most organs. The body is a sac perforated with holes


  2. 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 flagellum.


Eumetazoa: Radiata


  1. 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.


  2. Phylum Ctenophora, the comb jellies, are transparent animals whose locomotion depends on eight rows of cilia

Bilatria: Acoelomates


  1. 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.


  2. 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.




  1. Phylum Rotifera is made up of tiny animals possessing a crown of cilia that draws food into the mouth
  2. 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 trichinosis.