No true tissue True tissue

Radial symmetryTwo tissue layers

Bilateral SymmetryThree Tissue Layers

Protostomes Deuterostomes

Phylum Annelida

• The annelids or segmented worms include earthworms, leeches, and polychaete worms.

• They can be easily identified by the repeating segmentation of their bodies.

• These worms can be found in freshwater and marine habitats as well as moist terrestrial areas.

• Annelides have both longitudinal and circular muscles which allow them to move with their distinct “stretch then bunch” motion.

Click to see video

Earthworm poop or “castings” are important for soil

composition and texture which enhances plant growth and nutrient absorption.

Australian giant earthworms can be up to 3m long!

Leeches, which are blood sucking parasites, have been used for medical purposes for centuries. They are making a comeback as a way to safely drain excess blood from internal wounds and reduce swelling.

Polychaetes have pairs of distinct bristles on

each segment. They are used for movement like in the common clam worm of New England.

Or for feeding like in tropical feather duster and christmas tree worms.

No true tissue True tissue

Radial symmetryTwo tissue layers

Bilateral SymmetryThree Tissue Layers

Protostomes Deuterostomes

Phylum Nematoda

• While over 25,000 species are known it is estimated that there are 20X that number (no one has bothered to look for them because who wants to study round worms?).

• Their bodies are unsegmented and tapered at the ends with a tough covering.

• Nematodes are extremely widespread and can be found in almost all habitats including as parasites in the body tissues and fluids of other animals.

• They only have longitudinal muscles which give them their distinct thrashing motion.

Click to see video

Nematodes have also been know to cause severe destruction in Bikini Bottom (see if you can detect the multiple biological mistakes in the animation – I count at least three not including the fact that worms can’t talk). Click to see video

Nematodes can be very small Or quite large

Common soil nematodeIntestinal nematode from a pig

While there are thousands of free living species of nematodes, they are most famous for being disgusting parasites.

In plants In fish In insectsIn fact farmers use “beneficial” nematodes for pest control.

And in humans – the pictures are too gross even for me.

On a redeeming note, one species of microscopic nematode, C.elegans has been used extensively as a animal study model and has played a role in helping us understand the role of gene control in animal development.

Killer Amazonian nematode

No true tissue True tissue

Radial symmetryTwo tissue layers

Bilateral SymmetryThree Tissue Layers

Protostomes Deuterostomes

Phylum Arthropoda• The largest and the most successful phyla, it

is estimated that there are 1018 individual arthropods living on the planet at any one time.

• With over 1 million identified species (that’s 2 out of every 3 known species, of any kind) they occupy every type of habitat.

• Arthropoda contains hexapods (insects), crustaceans, spiders, and millipedes.

• While they are a very diverse group, all arthropods have some key distinctive features:

A segmented body Jointed appendages

And a hard exoskeleton

Because they are such a large group we are going to break the arthropods down into their four major subphyla.

No true tissue True tissue

Radial symmetryTwo tissue layers

Bilateral SymmetryThree Tissue Layers

Protostomes Deuterostomes

Phylum EchinodermataStarfish, urchins, sand dollars, sea cucumbers

• Echinoderms are more closely related to humans than they are to other invertebrates.

• All echinoderms have a thin skin that covers a calcium based hard shell or plates.

• Many echinoderms can be identified by the fact that their bodies are divided into five sections.

Echinoderms appear to have radial symmetry which is weird given their placement on the phylogenetic tree. However, their larvae are bilaterally symmetrical like and it is thought that their radial symmetry in adulthood was a later adaptation.

A key feature of echinoderms is their water vascular system.

The water vascular system is hooked up to a pumping system which allows echinoderms to create strong suction in their tube feet. They use this suction for movement, prey capture, and feeding.

A starfish using its tube feet to pry open a clam.

It is a series of water carrying vessels that run throughout the body and end in tube feet.

However, if you think they are slow, benign

creatures, think again.

Click for video

Aren’t echinoderms so cute!

vessels

Tube feetTube feet

No true tissue

True tissue

Radial symmetryTwo tissue layers

Bilateral SymmetryThree Tissue Layers

Protostomes Deuterostomes

Phylum ChordataAnd so ends our time with the

invertebrates. Now we can get to some of the stuff we usually mean when we say “animal”.

Chordates are a diverse group, which you are probably more familiar with. However, some of the members of this group may surprise you as well. Not all chordates have a hard skeletal backbone like you may think, but there are some key features which all chordates share.

1. NotochordA hard, flexible rod that forms between the nerve chord and digestive tract. In many animals it is eventually replaced with a hard skeletal vertebrae. In humans parts of the notochord remain as disks between our vertebrae.

3. Pharyngeal slitsOtherwise known as gill slits. Maintained from our water dwelling invertebrate ancestors, this feature usually disappears during embryonic development in terrestrial chordates. In mammals the slits go on to develop into the parts of the middle ear.

4. Post anal tailA tail (or remnants of one) that comes after your anus.

2. Dorsal nerve chordA nerve chord formed along the back from a rolled fold of ectoderm during embryonic development. In all chordates it goes on to become the brain and spinal chord.

anus

Chordate evolution contains many compelling stories.

The story of how animals moved onto land.The story of the evolution of the vertebrate

animals we are so familiar with.

The story of the dinosaurs. And the evolution of our ancestors.

For these reasons we are going to

take a closer look at the chordates.

Stay tuned……..