Biology- Mr. Willets
Phylum Arthropoda / Phylum Echinodermata
Phylum Arthropoda is the largest group of animals. The most obvious characteristic of arthropods is the exoskeleton with jointed appendages (the word arthropod means jointed feet). The exoskeleton gives protection but it is not as thick as a shell so it weighs less. It is made of a material called chitin (this is also found in the cell walls of fungi). The jointed appendages (legs etc.) allow for reasonable movement. Still, the weight of the exoskeleton limits the size of the animal. Another problem with this type of skeleton is that it must be removed in order for the animal to grow. During this time, the animal is vulnerable and generally hides until it has grown larger and then made a new exoskeleton. This is called molting. In restaurants, soft-shelled crabs are crabs that have molted and not yet grown a new exoskeleton. There are 5 main kinds of arthropods.
The crustaceans include lobsters, crabs, shrimp etc. These generally have 5 or more pair of legs. They are mostly marine. Many crustaceans are small and planktonic. They serve as food for larger animals. The krill that some whales eat are in this group. Crustaceans can have a heavier exoskeleton than the other arthropods because they live in water where the buoyancy helps support their weight. Their exoskeleton is made of chitin with lime (calcium carbonate). Crustaceans are an important food for people.
The arachnids are a terrestrial group of arthropods that include spiders, ticks, mites, scorpions etc. All of these have 4 pair of legs. The spiders are able to spin silk which they use to make shelter, capture their prey and move from place to place. Spiders and some other arachnids have poison jaws that they can use to paralyze their prey. Spiders that make webs are called orb weavers.
Centipedes are arthropods that have 1 pair of legs on each segment of their body. They do not have 100 legs which is what "centipede" means. These animals are hunters- they have poison jaws and are fast moving. The eat earthworms and other things like that. The bite of some centipedes can be quite painful.
Millipedes have 2 pairs of leg on each segment of their body. Despite this, they are slow moving. They are grazers and can damage the plants in your garden. Centipedes eat millipedes.
The insects are the largest of the arthropod groups. There are more insects than any other kind of animal. All insects have 3 pairs of legs and many have wings. Insects are fast in spite of having open circulation. This is because oxygen does not use the blood to get to the cells. The oxygen travels through a series of tubes called trachea. The tracheal system has openings called spiracles. These are located on the sides of the insect. Many insects have indirect development and go through metamorphosis. The larva of a butterfly becomes a caterpillar. The larva of a fly becomes a maggot. Insects are successful for several reasons. They are small and do not require much food. The types of food they eat varies- some eat leaves, some feed on flower nectar and pollen, some feed on blood, some feed on plant sap, some eat other insects. This means there is not as much competition with each other for food. The ability to fly certainly helps find food and escape danger.
Some insects are good for us- the silkworm (a caterpillar) makes silk. Bees make honey and pollinate our crops. The praying mantis eats harmful insects. Others annoy or harm us: a bee's sting, a mosquito's bite (which might carry disease), termites eating our homes, locusts destroying our crops.
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The echinoderms are an odd phylum of animals. They are the only phylum that is 100% marine. They have radial symmetry with the body usually divided into 5 parts. This is called penta-radial symmetry.
They have suction devices called tube feet which they can use for movement and to get food. These require a supply of water to work and the system of tubes that bring water to the tube feet is called the water vascular system. The skeleton of echinoderms is much like an exoskeleton in that it covers and protects the entire body but it is flexible and located under the skin. Spines stick out from the skin. Echinoderm means spiny skin.
Phylum Echinodermata includes sea stars (formerly called starfish), sea urchins, sea cucumbers (which look like cucumbers but are not related), brittle stars and sand dollars. Echinoderms are slow moving and most are grazers. Sea stars can eat bivalves (clams etc) in an interesting way. They attach their arms to both parts of the shell using the tube feet. They then pry open the shell a small amount and turn their stomach inside out through the mouth and into the shell where they digest and absorb the clam. They then pull the stomach back in and go on their merry way. Oyster farmers do not like sea stars and used to hack them into pieces when they caught them. Unfortunately for the farmers, sea stars have good powers or regeneration. Now they just throw them up on the beach and let them dry out. Of all the invertebrate phylums we have studied, the echinoderms are our closest relatives.
Questions-Arthropods/Echinoderms
1) What kind of skeleton do arthropods have? ________________________ It is different from a shell because it weighs ______________ and is made of ______________.
2) The jointed legs of arthropods allow them to ______________ reasonably well in spite of the exoskeleton.
3) The exoskeleton of arthropods limits their ___________________ . Also, it must be removed in order to _____________ This is called _________________________
4) Give some examples of crustaceans- ____________________________________________ We use many crustaceans as ________________
5) The exoskeleton of crustaceans contains _______________ to make it stronger. Why do crustaceans have this and not the other arthropods? ____________________________________________________
6) What organisms besides spiders are arachnids? ________________________________________
Spiders are the arachnids that can make ________________ Spiders that make webs are called ________________________
7) What can the silk of spiders be used for? _____________________________________________
8)How do centipedes get their food? _________________________________ How do millipedes get their food? __________________________________
9) Insects get oxygen to their cells through tubes called ______________________ This means they can be ______________ even though they have ____________ circulation.
10) You can't "drown" an insect by putting its head underwater. Why not? ________________________
____________________________________________________________________________
11) In what ways are some insects good for us? ___________________________________________
in what ways are some bad? _________________________________________________________
12) The most obvious characteristic that separates the 5 kinds of
arthropods is the number of _________ that they have. Crustaceans
have _____________________________ Arachnids have _____________
Centipedes have ____________________________ millipedes have
__________________________
and insects have __________________
13) The name echinoderm means ______________________________
14) Give some examples of echinoderms ________________________________________________
15) The suction devices echinoderms have are called ___________________________ These require a supply of _____________ to work. This gets to them through the ____________________________ system.
16) Most echinoderms get their food by _______________________ Sea stars can eat ____________.
To do this, they must open the ________________. They then stick their ___________________ inside the shell and digest the animal.