Diversity of Life, Part 1: Laboratory Exercise on Animals.
Animal Diversity
Despite our familiarity with tetrapods (four-legged forms with backbones) by far
the greatest breadth of animal diversity is in the invertebrate forms. Humans, on the
lookout for lions, tigers, bears and organisms somewhat similar to themselves, tend to be
biased toward their own body plan. In fact, of the 35 or so phyla of animals, all
backboned forms belong to one, the Chordata.
Definition of Animals
In the classic Linnean classification of life, organisms were divided into only two
kingdoms, plants and animals. Microscopy, developmental studies, and modern research
have shown that this simple dichotomy does not adequately represent the diversity of
life. Euglena, for example, is a single-celled organism that swims but also possesses a
green chloroplast, typical of plants. Is it a plant or animal? Neither, it is a protist, part of
the protoctist kingdom in the five-kingdom classification system, which itself is an
inadequate categorization. Within this current recognized taxonomy, animals are defined
as multicellular organisms that develop from a zygote that becomes a blastula, the
characteristic early form of the animal embryo, presumably a shared derived character for
the kingdom. All animals are multicellular organisms that originate from a sperm-fertilized
egg, becoming a blastula. Animals are heterotrophic (although some autotrophic
photosynthetic symbionts can exist within them).
Overview
Animals tend to have many complex tissues with different cell types after the
embryonic stem cells differentiate. They are heterotrophic (i.e., consumers), meaning they
eat the tissues of other living or dead organisms. Most groups are motile, although as the
Euglena example suggests, motility alone is not sufficient for definition. And animals are
sexual organisms, developing from gametes (i.e., sperm and egg). (Even “asexual”
animals—e.g., all-female populations of whiptail lizards—have evolved from sexually
reproducing ancestors, and still undergo sexual processes—e.g., self-fertilization—within
their cells.)
Major trends in animal evolution
1) Development of symmetry. Although some animals are asymmetrical (e.g., sponges)
most tend to be symmetrical. Radial symmetry refers to circular or cylindrical symmetry
as found for example in starfish or jellyfish. Bilateral symmetry refers to organisms with a
single axis of symmetry, with each side a mirror image of the other. Examples are dogs and
butterflies.
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2) Body structure. Because they are consumers, food must be taken in and waste
excreted. Body structure can be divided into sac-like beings (the same opening for food
intake and excretion) or tubular (a mouth at one end, an anus at the other).
3) Development of a coelom. The coelom is a cavity between the gut tube and the body
wall. It evolved in animals with three layers (ectoderm, mesoderm, and endoderm) via
splitting of the mesodermal (now middle) layer. Many animals lack a coelom.
Pseudocoelomate animals possess a cavity, but it didn’t evolve from a mesoderm.
Coelomate animals have a true coelom, that is a body cavity lined with mesodermal cells.
Coelomate animals comprise two main groups, the protostomes and the deuterostomes.
In the protostomes the blastopore becomes the mouth, the second opening becomes the
anus, the coelom is formed by splitting (schizocoely), there is spiral, determinate cleavage
when the blastula goes from four to eight cells, the circulatory system is primitively
dorsal, the nervous system is primitively ventral. Examples include mollusks (e.g., clams),
annelids (a kind of worm), and arthropods (e.g., lobsters). In deuterostomes the
blastospore becomes the anus (as opposed to the mouth), the second opening becomes
the mouth, the coelom is formed via enterocoely, and there is radial, indeterminate cleave
during the four-to-eight cell division. Examples here include echinoderms (e.g., starfish)
and chordates (e.g., humans).
4) Segmentation. Animals tend to repeat parts of their body plan to form a whole.
Earthworms for example are composed of many similar segments with small alterations.
Animals are divided into about 35 or so phyla according to body plan
The animal phyla are listed below in alphabetical order, implying nothing about
animal relationships or organization.
Acanthocephala
Acoela
Annelida
Arthropoda
Brachiopoda
Chaetognatha
Chordata
Cycliophora
Cnidaria
Ctenophora
Echinodermata
Echiura
Ectoprocta
Entoprocta
Gastrotricha
Gnathostomulida
Hemichordata
Kinorhyncha
Loricifera
Mesozoa
Mollusca
Nematoda
Nematomorpha
Nemertina
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Onychophora
Pentastoma
Phoronida
Placozoa
Platyhelminthes
Pogonophora
Porifera
Priapulida
Rotifera
Sipuncula
Tardigrada
You should examine the Phyla of the Animalia web page for this course on the lab
computers (http://www.ldeo.columbia.edu/dees/ees/life/slides/phyla/animalia.html) and
well as their phylogenetic relationships on the www page for the next lecture (Lecture 6
at: http://www.ldeo.columbia.edu/dees/ees/life/lectures/lect06.html .
Lab exercise
1. Using the web pages above, identify the phylum to which the specimens in front
of you belong.
2. Draw pictures of your specimens in you write up and identify the characters that
support your assignment each specimen to a phylum.
3. Then produce a cladogram in your write up of those phyla based on the
cladograms for lecture 6. You should show in this cladogram only those taxa (and
branches) that directly relate to your specimens.
4. Do you think the characters YOU see in these actual specimens would produce a
cladogram congruent with the latter? Please explain your conclusions in your lab
write up.
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