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Comparing Invertebrates. There are some four million different kinds of animals and plants in the world. Four million different solutions to the problems of staying alive . David Attenborough.

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comparing invertebrates

Comparing Invertebrates

There are some four million different kinds of animals and plants in the world. Four million different solutions to the problems of staying alive.

David Attenborough

Ediacaran Period – 600 million years ago.First multicellular animals with bilateral symmetry and segmentation
ediacaran ocean scene this was the last period of the precambrian and led to the cambrian period
Ediacaran ocean scene. This was the last period of the Precambrian and led to the Cambrian Period.

Cambrian Explosion – 542 million years ago. Most major phyla appeared here. A fantastic and sudden appearance of hundreds of different species. In just a few million years animals had evolved complex body plans. They had acquired specialized cells, tissues and organs.


The two main forms of cnidarians, polyp and medusa have radial symmetry.Nomura’s jellyfish – 2 m, 450 lbs. Coral Polyps – like anemones


Cephalization – nervous tissue becomes concentrated on one end of an organism. This allows for organisms to respond to the environment in more sophiscated ways.

Segmentation – allows for specialization of different parts and minimal new genetic material as each segment is similar to the next.

Coelom – a fluid filled cavity between the digestive tube and the outer body wall. It serves as a buffer between the outer wall and the inner organs.


Intracellular digestion – takes place in the cytoplasm of the cells of the organism. This is found in molluscs, cnidarians and Poriferans.This is a view of the Giant Barrel Sponge – the largest member of the phyla porifera (pore bearing). It is a filter feeder.


Extracellular digestion – Enzymes catalyse the digestion of the food outside the cells and the molecules are absorbed into the blood or body fluids


Cnidarian digestion Flatworm digestionIn both animals there is one opening. So wastes will go out the same tube that the food went in.


Digestive tract – one way flow from mouth to anus. Specialized cells help to digest the food, move the food, absorb nutrients, get rid of wastes, making the whole system more efficient.

Porifera respiration. Sponge cells absorb oxygen directly from the water. They eliminate carbon dioxide the same way.
cnidarian respiration there are no respiratory organs both cell layers absorb oxygen from the water
Cnidarian Respiration. There are no respiratory organs. Both cell layers absorb oxygen from the water.

Cnidarian cnidoctye. An explosive cell containing harpoon like cnidawhich also has a toxin. Some species (portugeese man of war, sea wasp) are dangerous to people.


Box jellyfish (class Cubozoa) are cnidarians distinguished by their cube-shaped medusa. Some species produce extremely potent venom. Three are among the most venomous creatures in the world. Stings from these are extremely painful and sometimes fatal to humans.

Box Jellyfish

A stinging victim

Portuguese Man of War. This cnidarian’s nematocysts (cnidocytes) can also penetrate human skin and leave a painful rash.

Respiratory system. The purpose of this system is to bring oxygen to the cells of the body and to eliminate carbon dioxide from the body. All have a large surface area and all are moist. In the anatomy of a typical mollusc(a gastropod – snail) the paired gills can be seen.


Respiration in land invertebrates. Respiratory surfaces are covered with water or mucus. These organs are usually covered to minimize water loss. Sow bugs or wood lice are crustaceans that breathe through paddle shaped hind legs. (pleopodal lungs).


Insects breathe through a series of openings (spiracles) that lead to tiny vessels (tracheal tubes) that bring oxygen to the cells of the body.


Open circulatory systems. Blood is only partially contained within a system of vessels. One or more hearts pump blood through vessels into a system of spongy cavities (sinuses). Arthropods and most molluscs.


The cephalopods are exceptions to the other molluscs in that they have a closed circulatory system. Their blood is blue because of the pigment hemocyaninwhich is different from our pigment (hemoglobin)

Insects have an open circulatory system. The blood moves from the hearts into the body sinuses and is collected again near the head

Excretory system controls the loss of water and gets rid of nitrogenous waste. (ammonia) Flatworms use a system of flame cells to eliminate excess water. Ammonia diffuses directly through the animals skin.

Annelids like the earthworm change ammonia to urea in the nephridia and excrete it. This process helps to save water.

Some insects and spiders use Malpighian tubules to change the ammonia into uric acid. This is excreted along with the solid waste as a thick paste. This saves the animal a lot of water.


Cnidarians are the first phyla with a specialized nervous system. It is composed of a simple nerve net that helps to coordinate motion of the animal.


The more complex the animal the more developed the nervous system. From simple nerve nets there appear ganglia, lumps of nervous tissue, and finally a brain in the head that controls the nervous system.


Cnidarians, flatworms, this roundworm, annelids and certain molluscs have hydrostatic skeletons. These skeletal systems consist of a fluid filled cavity that provides support and a place for muscles to push.


Exoskeletons. In arthropods a outer, hard covering composed of the protein chitin. Sun spiders, like this one from the Las Vegas area have an exoskeleton made of chitin


Endoskeletons are structural supports located inside the body. Sea stars and other echinoderms have an endoskeleton made of calcified plates.


Sexual Reproduction – most invertebrates reproduce sexually during at least a part of the life cycle. In sponges sperm and egg combine to form a free swimming larva that settles and develops into a sponge.

Life cycle of a jellyfish1–3 Larva searches for site, 4–8 Polyp grows9–11 Polyp strobilates12–14 Medusa grows

Sexual planaria are hermaphrodites each animal possessing both testicles and ovaries. Each planarian gives and receives sperm. Eggs develop inside the body.


Nematode Sexual reproduction. Most species have a male and a female worm. The eggs may hatch inside the female or they are deposited outside.


Annelid sexual reproduction. Earthworms are hermaphroditic. Sperm will be exchanged and a cocoon will form around the clitellum were the eggs and sperm will unite. The young worms are like the adults.

Cephalopods like this cuttlefish are male and female. Fertilization is internal here but can be external with other speices.

Arthropod sexual reproduction. Most have males and females. Some external fertilization, but most internal fertilization. Most lay eggs though scorpions give live birth.

Echinoderm sexual reproduction. Male and females with the release of eggs and sperm externally and tied to the rhythm of the moon.

Asexual reproduction in sponges. Three ways: by fragmentation, budding and producing gemmules. Gemmules are like spores and are produced by dying sponges

cnidarian asexual reproduction polyps can reproduce by budding hydra commonly reproduces this way
Cnidarian asexual reproduction – polyps can reproduce by budding. Hydra commonly reproduces this way.
Flatworm asexual reproduction – Planaria will spontaneously pinch off a part of the tail, which will become another planarian

Round worms do not have asexual behavior, but many parasitic ones have very complicated life cycles that involve one or more intermediate hosts.


Annelid Asexual reproduction. Limited to polychetes. Earthworms (oligochetes) can regenerate a lost tail, but do not reproduce asexually. Sabellid worm in parchment tube budding.


Arthropod asexual reproduction. Barnacles are hermaphroditic. Some insects (aphids) can reproduce by parthenogenesis. Parthenogenesis involved the growth and development of embryos without fertilization.

Echinoderms have a great capacity for regeneration. Some species can reproduce by parthenogenesis and a few can reproduce asexually.