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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.

classification of animals

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:

* multicellular

* eukaryotic

* 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)

from http://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookDiversity_7.html

Nicole King explains “All animals, from sponges to jellyfish to vertebrates [animals with a backbone], can be traced to a common ancestor. So far, molecular and fossil evidence indicate that animals evolved at least 600 million years ago. The fossil record does not reveal what the first animals looked like or how they lived. Therefore, my lab and other research groups around the world are investigating the nature of the first animals by studying diverse living organisms….. Choanoflagellates are a window on early animal evolution. Both cell biological and molecular evidence indicate that choanoflagellates are the closest living relatives of multicellular animals.




Between 620 and 550 million years ago (during the Vendian Period) relatively large, complex, soft-bodied multicellular animals appear in the fossil record for the first time. While found in several localities around the world, this particular group of animals is generally known as the Ediacaran fauna, after the site in Australia where they were first discovered.

The Ediacaran animals are puzzling in that there is little or no evidence of any skeletal hard parts i.e. they were soft-bodied organisms, and while some of them may have belonged to groups that survive today others don’t seem to bear any relationship to animals we know. Although many of the Ediacaran organisms have been compared to modern-day jellyfish or worms, they have also been described as resembling a mattress, with tough outer walls around fluid-filled internal cavities – rather like a sponge.


A new study mapping the evolutionary history of animals indicates that Earth’s first animal–a mysterious creature whose characteristics can only be inferred from fossils and studies of living animals–was probably significantly more complex than previously believed… the comb jelly split off from other animals and diverged onto its own evolutionary path before the sponge. This finding challenges the traditional view of the base of the tree of life, which honored the lowly sponge as the earliest diverging animal. “This was a complete shocker,” says Dunn. “So shocking that we initially thought something had gone very wrong.”

But even after Dunn’s team checked and rechecked their results and added more data to their study, their results still suggested that the comb jelly, which has tissues and a nervous system, split off from other animals before the tissue-less, nerve-less sponge.

The presence of the relatively complex comb jelly at the base of the tree of life suggests that the first animal was probably more complex than previously believed, says Dunn.



Is this possible? for this to be true, it would seem that complex structures – neurons – have evolved twice! Independently? See here for more amazing details:

Did neurons evolve twice? The curious case of comb jellies
sponge stat 5000 species comic

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