The Evolutionary History of Biological Diversity
An overview of Animal Diversity

2.7 An Overview of Animal Diversity

Objectives

By the end of this module,

 

Animals are heterotrophs, meaning they cannot fix carbon, and require organic carbon compounds for growth. By consuming reduced carbon compounds, heterotrophs can use all the energy they obtain from food in their growth and reproduction (unlike autotrophs that must use some of their energy for carbon fixation). However, heterotrophs are unable to make their food, and can die from an absence of food. All heterotrophs feed directly or indirectly on other living things. Unlike animals, fungi are heterotrophs that digest food outside of their body before taking up the nutrients through absorption, whereas animals ingest their food and then digest it internally

Animals are multicellular organisms. Their cells are distinguished from those of other eukaryotes, most notably plant cells, by their lack of cell walls and chloroplasts. Due to the lack of a rigid cell wall, animal cells can adopt a variety of shapes. A phagocytic cell can even engulf other structures. Because they lack cell wall support, animals have extracellular proteins, the most abundant being collagen.

Animals have many cell types that are not found in other multicellular organisms, including muscle cells and nerve cells. They also have unique tissue types, including connective, muscle, nervous, and epithelial tissues.

 

Variety of Body Plans of Animals

Most animals develop from three primary germ layers (termed triploblastic), with lower forms developing from two (diploblastic). Animal cells lack cell walls, and their bodies exhibit some form of body symmetry:

Animals exhibiting different types of body symmetry

Figure 1. Animals exhibit different types of body symmetry. The (a) sponge, shown in a peach color, is asymmetrical and has no planes of symmetry, the (b) sea anemone, shown in white, has radial symmetry with multiple planes of symmetry which are shown in blue, and the (c) browngoat has bilateral symmetry with one plane of symmetry, also shown in blue.
"This work" by , OpenStax College is licensed under CC BY 3.0

 

Animals are also classified according to the type of body cavity they possess (body cavity - a space between the digestive cavity and body wall) and also according to embryological development (protostomes and deuterostomes). What type of digestive tract is another characteristic observed in animals:

• Incomplete: have only one opening which serves as both mouth and anus

• Complete: have both a mouth and an anus

Cephalization or the presence of a head and localization of sensory organs is also an important characteristic to observe. Most animals are divided into two groups, invertebrates, and vertebrates. Most animals (97%) are invertebrates and are termed such as they lack a backbone.

 

 

 

AnimalsRelativeNumbers

Figure 2. This diagram is a pie chart showing the contribution of each animal phylum to the total number of animal species. Beginning at the top of the key and indicating the smallest slice of the pie is the Bryozoa shown in dark blue. The next slice is the porifera shown in red. The third slice represents 14 minor phyla and is shown in an olive green. The next and slightly larger piece is the echinodermata shown in purple. Continuing clockwise around the pie chart is the cnidaria shown in bright blue. Next come the annelida shown in orange. The next slightly larger slice is the nematoda shown in bluish-grey. Again moving clockwise, the next section is the platyhelminthes shown in a pink. The next increasingly larger slice is the chordata shown in pale green. The second largest slice is the mollusca shown in light purple. Finally the largest group taking up most of the pie chart is the arthropoda shown in blue.
"Animals Relative Numbers" by
Nick Beeson, Wikimedia Commons is in the Public Domain

 

Animals are generally considered to have a common ancestor, evolving from a flagellated eukaryote. Their closest known living relatives are the choanoflagellates, which are collared flagellates that are morphologically similar to the choanocytes of certain sponges. Animals form the kingdoms Animalia orMetazoa.

Eumetazoa is a clade comprising all major animal groups except sponges and placozoa. Characteristics of eumetazoans include true tissues that are organized into germ layers and an embryo that undergoes a gastrula stage. The clade contains Ctenophora, Cnidaria, and Bilateria.

The Bilateria include all animals with bilateral symmetry, i.e., they have two planes of symmetry. The Bilateria comprise a major group of animals and includes the majority of phyla. The most notable exceptions are the sponges, which belong to Parazoa, and cnidarians, which belong to Radiata. The parazoa group is now considered paraphyletic. It is not included in most modern cladistic analyses. For the most part, bilaterians have bodies that develop from three different germ layers. Nearly all are bilaterally symmetrical. The most notable exception is echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.

 

Bilateria tree  

Figure 3. The diagram shows the phylogenetic tree of animals. On the left side you have metazoa (animals) this then branches into parazoa (no tissue) and eumetazoa (specialized tissues). The only member of the parazoa group is the sponges. The eumetazoa then divides into the radiata (radial symmetry, diploblastic) and bilateria (bilateral symmetry, triploblastic). The radiata group divides once more into the cnidarians and comb jellies. Bilateria splits in to the protostomia and dueterostomia, the later further divides into chordates and echinoderms. Protostomia divides into ecdysozoa and lophotrochozoa. The ecdysozoa splits into arthropods and roundworms. The lophotrochozoa then divides into five categories: flatworms, rotifers, ribbon worms, annelids, and mollusks.
"Phylogenetic Tree of Animals" by OpenStax College is licensed under CC BY 3.0

 

Self-Assessment

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Glossary

Below is a list of terms identified in this module.

Bilateral Symmetry
type of symmetry in which there is only one plane of symmetry, so the left and right halves of an animal are mirror images
Choanocyte
(also, collar cell) sponge cell that functions to generate a water current and to trap and ingest food particles via phagocytosis
Ecdysozoa
clade of protostomes that exhibit exoskeletal molting (ecdysis)
Eumetazoa
group of animals with true differentiated tissues
Eumetazoa
group of animals with true differentiated tissues
Gastrula Stage
stage of animal development characterized by the formation of the digestive cavity
Germ Layer
collection of cells formed during embryogenesis that will give rise to future body tissues, more pronounced in vertebrate embryogenesis
Lophotrochozoa
clade of protostomes that exhibit a trochophore larvae stage or a lophophore feeding structure
Parazoa
group of animals without true differentiated tissues
Radial Symmetry
type of symmetry with multiple planes of symmetry, with body parts (rays) arranged around a central
disk

Glossary terms from OpenStax College Concepts of Biology licensed under CC-BY 3.0.

 

 Grant and Other Information

Creative Commons License

This work by Community College Consortium for Bioscience Curriculum is licensed under a Creative Commons Attribution 3.0 Unported License.
Text from BioBook licensed under CC BY-NC-SA, Boundless Biology Open Textbook licensed under CC BY SA, and OpenStax College licensed under CC BY 3.0. Modified by Courtney A. Harrington, PhD for C3BC.
Instructional design by Nicole P. Lipscomb, M.S., Helen Dollyhite, M.A., Caroline Smith, M.S., Irene Yee Chief, Ph.D. and Courtney A. Harrington, Ph.D. for c3bc.
Subjext Matter Expertise by Tiffany Davis for c3bc.
Media and interactive objects by Joe deCastro and Lucious Oliver, II for c3bc.

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