The Evolution, Structure, & Function of Animals - NOTES
Animals are eukaryotic, multicellular, heterotrophs with no cell walls.
Other characteristics of animals
•motile – capable of movement
•diploid – 2 copies of each chromosome in every cell, one from “mom” and one from “dad”
•reproduce sexually – gametes (sperm and egg) are produced by meiosis and fuse at fertilization (when a single sperm enters an egg) producing offspring with unique combinations of chromosomes
•creates genetic diversity
•cephalization – trend toward sensory organs (like a brain or a bundle of nerves) being clustered at the anterior end (front) of the animal
•segmentation – repeating body sections visibly present in most animals at some point in development
Invertebrate or Vertebrate?
•invertebrate – organism without a backbone of vertebrae
–sponges, cnidarians (jellyfish), worms, mollusks, arthropods, echinoderms (starfish), invertebrate chordates
•vertebrate – chordates with backbones composed of vertebrae
-fish, amphibians, reptiles, birds, mammals
Symmetry
•Animals with true tissues can then be divided into those that are
–radially symmetrical (radiata)
•Cnidaria (jellyfish) & Ctenophora (comb jellies)
–bilaterally symmetrical (bilateria)
•all other animals
*sponges are asymmetrical (have no symmetry)
Reproductive Terms
•hermaphrodite – adaptation to sessile or slow moving lifestyle in which a single organism produces both male and female gametes; usually sperm are exchanged resulting in both organisms retaining or shedding fertilized eggs
•oviparous – eggs* are laid in a nest or in the ground and young are hatched
–Many fish, amphibians, reptiles, and all birds;
–If the eggs have a hard, waterproof shell, fertilization was internal
•ovoviviparous – eggs are fertilized inside the parent and hatch inside the parent; young are born alive
–Some fish & reptiles
•viviparous – internal fertilization with live born young; fertilized egg becomes an embryo which implants in a uterus and develops into a fetus
–Some fish, reptiles (?), all marsupials and placental mammals; humans are placental mammals
Evolution
The traditional divisions of animal phyla are based on the following:
true tissues
symmetry
body cavity (coelom)
embryonic development pattern
True Tissue
•sponges (Phylum Porifera) are the only animals that do not have true tissues.
•simpler phyla, such as jellyfish (Cnidaria), have 2 layers of tissue – called diploblasts
–endoderm – innermost layer such as the cells of the digestive tract
–ectoderm – outermost layer such as the cells of the skin (dermis)
•all other phyla have 3 layers of tissue – called triploblasts
–mesoderm lies between the endoderm and ectoderm; gives rise to muscle tissue
Diploblast Triploblast
Body Cavity (Coelom – pronounced seelum)
•bilaterally symmetrical animals can be divided into three groups (which are important in discussing characteristics of animals but do not reflect true evolutionary relationships) based on the presence or absence of a body cavity
–acoelomate – no body cavity
–pseudocoelomate – body cavity not completely lined by mesoderm
–coelomate – body cavity completely lined by mesoderm
Most scientists agree that animals were coelomate and that pseudocoelomate and
acoelomate evolved based on adaptations to habitat/ niche
In humans, the coelom is where our organs are:
liver, stomach, intestines, etc.
Embryonic Development Pattern
•bilateral/coelomate animal embryos divided in one of two patterns
–protostome (first mouth)
•mollusks
•annelids (segmented worms)
•arthropods
–deuterostome (second mouth)
•echinoderms
•chordates
•The first criteria described is the CLEAVAGE PATTERN
–cleavage – division of the zygote into a solid multicellular ball by mitosis; recall the term cleavage furrow from our study of mitosis
–the cells are basically undifferentiated (all the same; stem cells that are totipotent)
–the embryo doesn’t change size from zygote to morula – the cells just get smaller
•the second criteria is how the coelom (body cavity) forms
–schizocoelous – 2 small pockets of mesoderm (think muscle) split and expand until they fill the cavity, forming the coelom
–enterocoelous – folds of the archenteron (think gut cavity) fold down and expand until they fill the cavity, forming the coelom
•The third criteria is the formation the mouth
-protostome – the blastopore becomes the mouth
-deuterostome – the blastopore becomes the anus and the mouth forms second at the opposite end of the embryo axis
Amniotic Egg
•among vertebrates, one of the keys to fully adapting to a terrestrial life-style is the development of the amniotic egg
•often described as a self-contained pond, it is water-proof but not air-tight, allowing the exchange of oxygen and carbon dioxide with the air
•helps the embryo maintain homeostasis by regulating nutrients, waste, and gas exchange with specialized extraembryonic membranes
The allantois holds metabolic wastes produced by the embryo.
The chorion and the allantois exchange gases between the embryo and the air.
The amnion protects the embryo in a fluid-filled cavity that cushions against mechanical shock.
The yolk sac provides nutrients via blood vessels; other nutrients are stored in the albumen (egg white).