Animals are organisms classified in the kingdom Animalia.
in the diagram below, notice bacteria, fungi, plants – and animals. The term animal refers to a specific biological grouping, called a kingdom.
Within the animal kingdom, animals are divided into various sub-groups.
Vertebrates: animals with a backbone – birds, mammals, amphibians, reptiles (*), fish.
(*) Reptiles, well, they’re kind of not really a meaningful group anymore – we’ll learn about that later.
Invertebrates: animals without a backbone –
Coelenterata – comb jellies, coral animals, true jellies (“jellyfish), sea anemones, etc.
flatworms – Planarians, flukes and tapeworms
Annelids – over 17,000 modern species including ragworms, earthworms, and leeches.
Mollusks – clams, oysters, octopuses, squid, snails
Arthropods – millipedes, centipedes, insects, spiders, scorpions, crabs, lobsters, shrimp
Sea sponges (not on the diagram below)
arachnids – 100,000 species of spiders, scorpions, ticks, mites, etc.
Crustacean – 17,000 species of crabs, lobsters, crayfish, shrimp, krill and barnacles.
Insects – over a million different species!
Myriapoda – Over 13,000 species of centipedes and millipedes
You’re an animal! The biological definition of “animal” includes all mammals, including humans. Specifically, you are in the mammal family.
Typical animal cell, in three dimensions
Here is a typical animal cell, as drawn in 2D on a Regents Exam.
1. lysosome, 2. endoplasmic reticulum, 3. chromosome (DNA)
4. golgi body (apparatus), 5. vacuole, 6. mitochondria
7. ribosome, 8. nucleolus, 9. nucleus
10. centrioles, 11. plasma membrane 12. cytoplasm
How are cell diagrams typically drawn:
#1 – generally, lysosomes are illustrated as “shaded-in” circles or ovals.
#2 – Transportation channels running through the cytoplasm
#3 – Chromosomes are made of genes, which are made of DNA nucleotides.
#4 – stacks of membranes (like pancakes)
#5 – an “empty” oval or circle
#6 – recognize mitochondria by the zig-zag line drawn in them. these are where respiration reactions occur.
#7 – very small dots, sometimes on the endoplasmic reticulum, sometimes out floating in cytoplasm
#8 – drawn as a dark circle inside the nucleus
#9 – generally depicted as the largest “shaded-in” circle inside the cell
#10 – drawn in pairs, two cylinder-like structures at right angles to each other
#11 – the outer boundary
#12 – the watery fluid that everything else floats in
Animals are characterized by being:
* Do not have rigid cell walls (those would be found in plants, or bacteria)
* body plan becomes fixed as they develop, although some undergo metamorphosis.
* motile (can move spontaneously and independently, at some point in their lives)
* heterotrophs – must eat other organisms, or their products, for sustenance.
* Nearly all animals have bodies differentiated into separate tissues. **
** except for the simplest animal-like organisms, e.g. sponges
Evolution of the first animals
Animals probably evolved from marine protists, although no group of protists has been identified from an at-best sketchy fossil record for early animals.
Cells in primitive animals (sponges in particular) show similarities to collared choanoflagellates as well as pseudopod-producing amoeboid cells.
Multicellular animal fossils and burrows (presumably made by multicellular animals) first appear nearly 700 million years ago, during the late precambrian time….
All known Vendian animal fossils had soft body parts: no shells or hard (and hence preservable as fossils) parts.
Animals in numerous phyla appear at (or in many cases before) the beginning of the Cambrian Period ( 540 million years ago)