INVERTEBRATES

INVERTEBRATES -
ONE OF TWO MAJOR ANIMAL GROUPINGS

There are two basic groups of higher animals. They are vertebrates and invertebrates. While both have advanced through the processes of evolution, there is one fundamental difference. Invertebrates do not have backbones. Both groups are in the Kingdom Animalia, but their bodies are organized differently. What makes invertebrates different? All invertebrates share common traits. At the bottom of the invertebrate world are the sponges. Sometimes they don't fit in but they are still part of the group. Here's the nice and neat little list. 

(1) They are multicellular. It's more than being a colony of individual cells. The cells are working together for the survival of the organism. All of the cells have specific duties and responsibilities. 

(2) No backbone. We already talked about this one. That's the whole definition of invertebrate, no vertebrae. 

(3) No cell walls. When we talked about plants, we always mentioned cell walls. Invertebrates don't have them. Remember that even if none of them look like animals, they are. Being an animal means you have no cell wall. 

(4) Here are a few that have the qualifier "most" attached. That means not all of them have the trait, but most do. Most of them have tissues (not sponges) that are specific organizations of cells. Most of them reproduce sexually (not asexually). That means twogametes combine to form a new organism. Those gametes come from separate organisms (male and female). 

Most invertebrates can move. Even sponges move when they are very young and very small. Once they settle down they don't move anymore. Other invertebrates likelobsters and insects move around their whole lives. Most invertebrates are organized in a way called symmetrical. Symmetrical organization means when you can draw a line down the middle of the organism and the two sides look like mirror images. Draw a line down the middle of yourself and one side looks like the other side. If you draw a line down the middle of an octopus you would find two sides with equal parts. Remember we said most? Sponges and some coral are not symmetrical. 

(5) Invertebrates can't make their own food. Scientists use the word heterotrophic. Heterotrophs feed off other things to get their energy. Plants are autotrophic. They make their own food. Being heterotrophic is one of the main characteristics of being an animal. We eat things, whether it is plants or other animals. That's just the way the world works.

SPONGES

THE FIRST INVERTEBRATE

Sponges are kind of like the exception to the rules of invertebrates. They are so basic that they have not developed many of the characteristics you might think of when you think of animals. Most sponges are found in seawater, only a few in fresh water. You may have even seen a couple around your bathtub! The ones around your bathtub might look like sponges but they are usually man made. When we talk about sponge structure, remember all the holes. They are very important. 

STAGES OF LIFE

There are two basic forms in the life cycle of a sponge. Most sponges live their lives attached to a reef. They don't move around. There was a time in their lives when they were little larvae that they were swimming around the water all by themselves. The word larva is another way to describe them when they are babies. Baby sponges don't look like adult sponges, so scientists use another word. Once the larvae land on a piece of rock, they take root (so to speak) and that's that, forever anchored. 

Sponges are really just a bunch of specialized cells working together to help the entire organism survive. Sponges do not have nervous systems, so they don't react to the world around them. Sponges are in the shape of a big "U." On the outside of the U are protective cells, but on the inside are these very special cells with little flagella (wildly whipping tail structures). Those flagella are constantly moving and keeping the water circulating inside of the sponge. Water is sucked in through holes/pores in the side of the sponge. We told you to remember the holes. When the water moves through the sponge, tiny food particles are filtered out of the water by the flagella. Then the water gets pushed out of the sponge through a hole called an osculum. That's basically the life of a sponge. Suck the water in, filter out the food, and send the water out. 

THE CHOANOCYTE

The entire life of a sponge revolves around one type of cell. We already told you about flagella. Those flagella are part of a cell called achoanocyte. It's a cell that has three basic parts: flagella, collar, and cell body. Sponges use the flagella to move when they are larvae. The flagella and collar work together to gather food. Sponges even use the choanocyte when it's time to reproduce. Wow! It's a really busy cell.

STARFISH AND URCHINS

SPINY SKINS

These organisms are great. They are also very beautiful. Starfish, urchins, and sea cucumbers are all grouped as echinoderms. Echinoderm? What does that mean? "Echino" means spiny and "derm" means skin. You may have heard of your epidermis. That's your outside skin. The creatures in this group all have some type of spiny structure on the outside. 

TUBE FEET

What makes them so special? Can you say that you have tube feet? They can. If you ever turn a starfish over you will see hundreds of little tubes on each arm. Those tubes attach to an object, suck in, and attach to help the creature move. Those tube feet are also quite handy when it's time to eat. Starfish are hunters, moving around the rocks looking for food. Sometimes they come across a bivalve (mussel or clam). The starfish doesn't want to eat the shell. It wants to eat the organism inside. They use those little tubes to attach to the shell. Since the suction is so strong, that can slowly open the bivalve and eat the soft part of the mussel inside. 

AND A VASCULAR SYSTEM

The other big idea that is not so easy to see is that starfish have something called a water based vascular system. You have blood that transports nutrients all over your body. Echinoderms circulate water throughout their bodies. The system not only transports molecules, but also works with muscles to walk and move. The canals of their vascular system are found throughout their entire body. 

We already talked about starfish moving around and hunting down bivalves for dinner. In the echinoderms, you'll also discover that everybody seems to eat in a certain way because of their body types. Starfish move around. They have a lot of strength. They are able to go after living creatures. Urchins are a little different. They don't move around so well. They also don't have arms to help them grab food. You'll find that urchins are the grazers of the echinoderms. Like cows eating grass, they eat little pieces of food and algae. Finally, you have your sea cucumbers. Round, squishy and not very impressive. They sit on the bottom of the ocean and eat. More specifically, they eat whatever you find down there.

ANEMONE AND CORALS - THE CNIDARIANS

We'll start by explaining that anemone, coral, and jellyfish are all related. We broke them up into two different sections because of their body types. They are all from thePhylum Cnidaria. Comb jellies are a side step away from jellyfish. They are in the Phylum Ctenophora. They look similar, but are different in some important ways. 

ANEMONE

Specifically, anemones are considered the cnidarians that look like flowers. They have a central body and dozens of tentacles waving in the water, waiting for prey to pass by. You can find them alone or in groups. If you ever go to tide pools, you will see them all over the rocks. You will find anemone wherever you find a lot of fish and healthy water. 

CORALS

Corals are different from anemone because they have a skeleton of sorts. Anemones are squishy and basically filled with water. Corals create a hard skeleton of calcium carbonate. You might see that skeleton for sale in stores. When you find living coral and look very closely, you will see thousands of living cells waving about in the water. We should probably mention, although obvious, that coral do not go anywhere. Once they settle down, that's it. You might find coral alone or thousands of them together forming entire reefs like the Great Barrier Reef near Australia.

JELLYFISH

MORE CNIDARIANS

We'll start by explaining that anemone, coral and jellyfish are all related. We broke them up into two different sections because of their body types. They are all from the phylum Cnidaria. Comb jellies are a side step away from jellyfish. They are in the phylum Ctenophora. They look similar, but are different in some important ways. 

we discussed the basic ideas of Cnidarians when we talked about anemone and coral. Jellyfish are different in that they are not anchored to anything. Where jellyfish and coral are considered polyp shapes, jellyfish are in a medusa shape. Medusas are free swimming shapes that have their tentacles on the bottom of their body (not facing up like anemone). Since they move around, that newly evolved nervous system comes in handy. 

The main body of a jellyfish is called the bell. That bell moves through a coordinated contraction that forces water out and thrusts the jellyfish forward. They spend their whole life cycle floating with the currents and capturing prey in their tentacles. They also have those stinging cells (nematocysts) on the tentacles to paralyze fish and then eat them. 

COMB JELLIES

Comb jellies are not true jelly fish, they are kind of one step up. The big differences...

- Comb jellies use cilia flapping on their sides to move. They do not contract a bell like jellyfish.

- Comb jellies use cells called colloblasts to capture their prey. These cells stick to the prey, not harpoon them.

- They have two (2) holes. While true jellies have a sealed bell with only one opening (like the one opening of an anemone), comb jellies have a small opening at the top of their body and one at the bottom so that water can flow through them.

- They often have plates for protection on the outside of their bodies. The plates help comb jellies maintain their shape. Since they don't move by contracting their whole body, it helps keep them a little more streamline.

CLAMS AND SNAILS -

THE MOLLUSKS

Some people are saying "Yummy" right about now. Not us. Clams, snails, slugs, mussels, oysters, and clams are all considered to be mollusks. Squid and octopi are also mollusks but we put them on a different page because they are very cool and actually somewhat intelligent. Clams are not known for their intelligence. Mollusks are known to be the first animals to have developed gills. 

SNAILS AND SLUGS

You can find them on land and in the ocean. Bivalves are usually only found in water. What are some characteristics of gastropods? They have tentacles on their heads. Think about a snail for a moment. It has two stalks for the eyes and two shorter ones towards the front of the head. It can actually move those to look around. They aren't tentacles like an octopus. 

Gastropods might or might not have shells. Slugs are basically snails that have lost their shells over years of evolution. Snails have those big spiral shells that can come in a variety of shapes and sizes. They are usually made out of a compound calledcalcium carbonate. The shells have little compartments inside where the creature can place its digestive system. 

SOME WITH TWO SHELLS

Let's talk bivalves. This is the place where you'll find clams and mussels. These invertebrates have two shells. The shells are connected at the base by little ligaments that the creatures control. They are able to open and close the shells for protection, to help them move, or to open up and eat. These creatures even have tiny eyes. They will see you coming if you are trying to sneak up on them.

FLATWORMS - UMMM. THEY'RE FLAT

We have to run out of steam there. But that's the one obvious connection between all of the worms in this group. Sure we can tell you that there are over 20,000 known species. Most of the species are parasitic. They are super-primitive organisms that were the first to develop mesoderm. The big thing you should remember is that they are flat. There may be one more fact to remember. They are found in every environment that has water. Many scientists believe there are hundreds of species we will still discover. 

BASIC TYPES

You'll learn about a few classic examples of flatworms in your classes. Tapewormsare always a nasty example. They live in intestinal tracts of many species. Cats, dogs, and even you could wind up with tapeworms. You'll also hear about flukes. It's another parasitic species that has a close relationship with man and other mammals. They are suckers in the flatworm family. Planaria are harmless flatworms that are often used in your biology labs. They are freshwater flatworms that have a head like an arrow. They're black with two white spots on their heads. Those spots are not eyes, but they do sense light. Flatworms have very simple nervous systems. 

WHERE WILL YOU FIND THEM?

If you find water, there's a good chance flatworms will be around. Most are very small and not obvious immediately. Flatworms get most of their oxygen through diffusion. Since they have no specialized circulatory system, their flatness gives them a greater surface area to absorb more oxygen. There are species in many freshwater and saltwater environments as well as inside larger organisms. 

While we made them seem easy to find, it's sometimes very hard to track them down. Since many flatworms are parasites, they have lifecycles in different hosts. Eggs many be deposited in the feces of one species. Another organism might eat those eggs and development begins in the digestive system. The flatworm might then mature in the muscle tissue. Flatworms can get around. 

SOME STRUCTURE STUFF

Although really simple, flatworms have some advanced structures compared to some other animals. They were the first species to develop mesoderm. That mesoderm tissue develops into organs and muscles as the organism grows. They also have simple nervous systems and sensory organs. 

While they don't have a respiratory system or circulatory system to speak of, they do have a neat little digestive system. Most species of flatworms have no anus and a cavity with only one opening. While not all species do, many flatworm species use a mouth to eat. The only problem with no anus is that the stuff you don't digest has to go out the mouth when you're done. Other materials that need to be removed from the system can be excreted through specialized cells called flame cells. Flame cells are the first appearance of excretory ducts in organisms. Flame cells have cilia that beat and filter out materials that the flatworm wants to get rid of.

NEMATODES - WORMS THAT ARE ROUND

So you've got your segmented worms and your flat worms. There are more worms out there called nematodes. Nematodes are also called round worms. You might not know (but now you do) that there are more roundworms in the world than any other creature (we are referring to multicellular creatures). They are small and they are all over the place. If you are having problems imagining that, remember that there is an entire bottom of the ocean where insects and fish can't live. Guess who can? 

GOOD TO HAVE A GUT

We like telling you what makes each type of animal special. Nematodes are also special. While you may see a small smooth worm, scientists see creatures that have a gut that is complete. Mouth, lips, gut, and a hole when basic digestion is done. That's pretty neat (if you're into nematodes). That complete gut was one of the first steps to organizing physiological systems. It is one step up from organisms that only have tissues. 

DID WE MENTION THEY ARE ROUND?

They do not have specialized circulatory systems like annelids. And guess what... They are round! You knew that. Nematodes are round and have bilateral symmetry (both halves look the same). They also have special muscles that move down the length of that entire round body. Annelids (segmented worms) are different in that their muscles just work in each segment. 

ALL AROUND YOU... AND IN YOU

Where do you find them? We should say... Where don't you find them? Nematodes act like predators, hunting down other creatures. They are parasites, surviving at the expense of a host. The best example of nematodes as parasites is a disease calledTrichinosis. Some are also herbivores, eating plant material and algae. How do they get so many places? It helps that so many species of nematode are very small. You need a microscope to see most of them.

ANNELIDS - WORMS WITH SEGMENTS

Ooooh. Worms. Walk around school after a rainstorm and you will have seenannelids. Earthworms are the classic example of annelids. Annelids are all of the segmented worms. Earthworms have little sections. 

BODY PARTS IN SECTIONS

They are special. The whole segmentation thing is the big one. They actually have loads of body parts that are duplicated in each segment. If one segment is damaged, some annelids can go on living. You are not segmented. You have one of everything. If you lose something... Too bad. They also have something called a closed circulatory system. It doesn't seem that amazing because you have one. However, as far as developed creatures, annelids were one of the first. They circulate nutrients and compounds through their segments using tubes. Other creatures with open systems just let everything move around on its own. Closed circulatory systems are more efficient. 

MORE EFFICIENT MOVEMENT

Living on land brings several problems. When you live in the ocean or fresh water like an octopus or jellyfish, moving isn't much of a problem. Specifically, gravity isn't such a big issue. When you're on land, you gotta move. You gotta get to food. You gotta hide from predators. You gotta find other worms to reproduce. 

Annelids are able to move around by contracting their little segments. They have parts called setae. Setae are two pairs of hairs on each segment. Those hairs help some annelids (earthworms) get a grip on the soil. They are able to move through the soil easier with those setae. Those setae help other annelids called tubeworms grip the inside of their tubes so that they can move in and out more efficiently. 

When you look at all of the creatures of the world, remember this... It's all aboutefficiency. That's what helps you, your species, and your world survive. If you are a parasite and you kill your host before you reproduce, you're out of luck. It's not efficient. You have an advantage when you are efficient. That advantage is what makes life easier for you. And, if you can pass that advantage on to your kids (better eyes for finding food), so much the better.

ARTHROPODS - INVERTEBRATE POWERHOUSES

We'll start with the big numbers. About 75% of all animal species are arthropods. There may be more individual worms in the world, but there are more different types of arthropod than any other group on the plant. You'll find insects, spiders, crabs, lobsters, barnacles, centipedes, and even hundreds of extinct species in fossils. 

BASIC STRUCTURE STUFF

You know the first one. Arthropods all have exoskeletons. Exoskeletons are hard outer shells made of chitin. While you have an endoskeleton, a crab has a tough shell that protects it from the outside world. Next on the list are the arms and legs. They have jointed appendages. That's what the name arthropod means. jointed leg. Inside those joints and exoskeletons are muscles that help the organisms move. 

Not all exoskeletons are the same. While they may all have chitin, a shell created by the epidermis, crustaceans have an extra layer that is calcified. That calcification makes it much sturdier and much heavier. Arthropods also have very advanced sense organs. You are probably familiar with the faceted eyes of flies and antennae on insects. Those are great examples of how arthropods are prepared to interact with the world. They also have open circulatory systems. These systems circulate nutrients throughout the inside of that exoskeleton so the muscles receive all the energy needed to move quickly. 

MAJOR CATEGORIES

There are four major types of arthropod. Three are found around you while one group is extinct. Trilobites are an extinct group that can be found in fossils around the world.Chelicerates include species such as spiders and horseshoe crabs. Uniramiansinclude centipedes, millipedes and the biggest group of arthropods. Insects.Crustaceans are the last category of arthropod. Crustaceans include crabs and lobsters. They are mainly aquatic species.

LOOKING AT ARTHROPOD STRUCTURE

So what makes an arthropod? Why is it different from a mammal? Does an arthropod have a skeleton? Yes, but it's on the outside and called an exoskeleton. You might be wondering how an arthropod can get bigger if it has a hard outer skeleton. Doesn't it outgrow the shell? Yes. It has to shake off the shell and let a new one grow every now and then. When an arthropod loses one exoskeleton to grow bigger, it's called molting. 

Do arthropods have legs to move? Yes again! They have special legs that are jointed. They usually have several legs, more that the four that mammals have. Do they think? That's a tough one. They have advanced nervous systems. They even have groups of neurons that think and help the organism hunt, move around, and find a mate. Do they sit back and think about watching television? Probably not. Can they learn? Yes. Not a lot, but they can learn new behaviors. It helps them adapt to new situations. 

METAMORPHOSIS

We have one more big idea to introduce about arthropod development.Metamorphosis is a process where you change your body shape as you grow. In insects, it can be very special. There are even different kinds of metamorphosis. 

COMPLETE AND GRADUAL

You may have learned about caterpillars changing into butterflies. That is a complete metamorphosis. Another complete metamorphosis happens when maggots turn into flies. Complete metamorphosis means that the entire body shape has changed (wormlike into insect with wings). 

The other type of metamorphosis is calledgradual metamorphosis. Grasshoppers are a good example of a gradual one. They start off as small odd-shaped grasshoppers with no wings. As time passes, they molt their exoskeletons and grow wings. It's not a big change like a butterfly.

INSECTS ARE EVERYWHERE

We've talked about arthropods with spiders and crustaceans, but the biggest category of arthropods is the uniramians with all insect species. There are about 26 orders in the Class Insecta and you will find species everywhere. There may be tiny little fleas and there may be monster mosquitoes in the jungle. There may be beetles digging in burrows under the ground or dragonflies zipping around a pond. Insects are everywhere. Even in a simple backyard garden you might find twenty species that include butterflies, aphids, bees, flies, beetles, and grasshoppers. 

BASIC STRUCTURE STUFF

With such a variety, where do you start? Can a class with so many species have any similarities? Yes. They are hexapods. That means they usually have six legs. While a deer might have four and a spider might have eight, insects usually have six legs. They have exoskeletons made of chitin. They usually have compound eyes. Compound eyes have a series of facets, each one acts like a little eye with its own parts. A fly's eye is a classic example of a compound eye. They also have complete digestive tracts. While it might not seem like a big deal to you, having a mouth and an anus is a big advantage for an organism. 

That's where our list of similarities will end. The fun stuff is in the specialized structures you will find on insects. Insects were the first organisms to fly. Several species developed wings of transparent chitin and flight muscles that contract quickly. Flying was a big advantage when species were developing. A whole niche was open for new species. Many insects have also developed toxins. Bees are an easy example of insects that can sting and poison you. Speaking of bees, many insects have developed different compounds beyond poisons. Insects can make wax, silk, andhoney. Some insects such as butterflies have antennae. The list goes on. 

METAMORPHOSIS

Insects go through a process of metamorphosis. It's not the same for all species. Grasshoppers go through incomplete metamorphosis that happens when it loses its exoskeleton as it grows. It is incomplete because a baby grasshopper looks basically like an adult grasshopper, just smaller. Other insects go through a complete metamorphosis. These insects include butterflies and have a complete change in body structure. Butterflies start as eggs and hatch into caterpillars. When the time comes, those caterpillars develop a protective case and transform into a butterfly. The beginning looked nothing like the end. 

HUMANS AND INSECTS

We love them and we hate them. Mosquitoes spread diseases across the world. Since they take in blood, small viruses and microorganisms can hitch rides and move from one organism to another. For every mosquito there is a honey bee that helps us pollinate crops and provides us with some types of food. While the bees might sting when they feel threatened, they make modern farming possible. 

Back to the negative. Insects can destroy our crops. Locust swarms can eat entire wheat fields or fruit flies lay eggs in our fruit. It's a constant battle to find a natural way to control insects and still allow them to survive. The thing you need to remember is that the insects don't want to hurt us or compete with us. They are just out there trying to survive. They look for opportunities and take advantage of them. An opportunity for an insect might have ladybugs eating aphids from our plants. Another opportunity might have termites eating the wood in your house. They just go where the food is.

COOL CRUSTACEANS

We love crustaceans, one of the big groups of arthropods. They have crabs, lobsters, shrimp, and barnacles. You'll find all of them living in the water or in very wet environments. One example of a crustacean that lives out of the water is a pillbug or sow bug. That's right. Pillbugs are related to lobsters. 

What makes crustaceans similar to other arthropods? They have special legs that are jointed. They usually have several legs, more than the four that mammals have. They also have advanced nervous systems and groups of neurons that work to help the organism hunt, move around, and find a mate. Crustaceans can also learn. They can learn new behaviors and adapt to new situations. 

WE'VE GOT CRABS!

They're right on the dinner table. We also eat lobsters and shrimp. Crustaceans are a big source of food for many animals. What do they eat? Stuff on the bottom of the ocean. Stuff that's floating around the ocean. Even though you might think of their big claws, crabs and lobsters don't run around and hunt down food. It's a lot easier to find a dead fish and pick it apart. It's even easier to be a barnacle (which doesn't move once it becomes an adult) and wave around a filter to grabs pieces of food from the water. Remember, intimidating looks don't always mean aggressive animals. But don't go and put your finger in those claws, they will break your finger as a defensive action. 

THICK EXOSKELETONS

What else makes crustaceans so special? Their exoskeleton is very special. Some of the parts have actually fused together to form one piece. If you look very closely, you will see the places where they meet. They have something called a cephalothoraxthat is actually several sections of exoskeleton fused together. They have also developed a shield for their head. An insect head is sitting out there and could be bitten off. Crustaceans have developed a shield to protect their eyes and head area. Some of them don't even have obvious heads. One example is a crab that has one big shell protecting its entire body.

SPIDERS AND SCORPIONS - MORE ARTHROPODS

Spiders are also arthropods. Arthropods are a big group of creatures. We divided up our pages because the creatures come in such a variety. You would never confuse a tarantula, lobster and a butterfly, but they all have similarities. 

ARTHROPOD REVIEW

Spiders are close to insects, but still very different. Like all arthropods, they have jointed legs and segments for their main body. To keep those legs and segments moving properly, arthropods also have nervous systems that are very advanced. What else? Many arthropods also have an exoskeleton. The exoskeleton is the hard outer shell. If you touch an insect, spider or crab you'll find it to be hard. If you look at a mammal, they are usually soft. If they aren't, their protection is a specialized piece of their skin (the skeleton is on the inside). But what makes these guys different? 

SPIDERS, SCORPIONS AND HORSESHOE CRABS... OH MY!

Ah yes... The subphylum Cheliceriformes. There are a lot of them. Over 70 thousand species. The big thing that sets them apart is the number of legs. Eight (8). Most of you probably know that spiders have eight legs. So do scorpions and horseshoe crabs. They usually don't have antennae either. They have complex eyes so they can find prey. Did we mention that many of the members of this subphylum are predators? We should. 

While you might find spiders in many locations around your house, scorpions are a little harder to find. Horseshoe Crabs are even more difficult to see. Horseshoe Crabs live in the ocean. You might find spiders and scorpions near the water, but they are not fond of living in the water. Some spiders can actually walk on water to capture their prey. It's a whole surface tension thing to keep them on top. 

Spiders have the ability to live in almost any environment while you will usually find scorpions in dry areas. We just saw a spider in our offices this morning. But here in California, we have many scorpion species found across or deserts and grasslands. We haven't seen a scorpion in our offices. Thank goodness.