What is this?
A crab.
How many legs does it have?
10
What kind of skeleton does it have?
An exoskeleton
Describe characteristics of its life cycle.
| Characteristic | Why you would know |
| It lays eggs. | As a general rule, all macroscopic animals except mammals produce eggs. However, it is much more complicated than this. See Modes of reproduction – Wikipedia |
| It has a larval stage and goes through metamorphosis before becoming an adult. | Analogous to other arthropods such as insects and spiders. |
| Sexual reproduction | Analogous to other arthropods |
| It molts its shell as it gets bigger | Required by its exoskeleton |
What are the levels of biological classification?
Kingdom, phylum, class, order, family, genus, species
What is the mnemonic for biological classification?
Kings play chess on fine glass sets.
How would you classify this animal?
It is an invertebrate, arthropod, crustacean and decapod. It is in kingdom Animalia, phylum Arthropoda (the largest phylum), and order Decapoda. It’s class, Malacostraca, is not one you are likely to recognize, but it is the second largest class within Arthropoda after the insects.
The motivation for this image is that it is an arthropod. We are most familiar with a relatively few species, mostly vertebrates. But vertebrates are only about 3% of all animals. Vertebrates belong to the subphylum Vertebrata within the phylum Chordata. There are at least 31 phyla (plural of phylum). All vertebrates belong to one phylum. All other phyla contain invertebrates. Arthropoda is the phylum with the largest number of species, mostly due to the insects. In the diagram below, the phylum Arthropoda is represented by the insects, arachnids, and crustaceans.
What kind of symmetry does it have?
It has bilateral symmetry.
What color is its blood?
Blue.
The blood of vertebrates contains oxygen-carrying hemoglobin which is made with iron. It is the iron that gives the blood its red color. Arthropods have an analogous oxygen-carrying protein called hemocyanin which contains copper instead of iron. The copper gives their blood (called hemolymph) a blue color.
| Vertebrate | Arthropod | |
| Circulating fluid | Blood | Hemolymph |
| Circulatory system | “Closed” Blood never leaves the blood vessels | “Open” Hemolymph is not confined to vessels, but comes in direct contact with the tissues |
| Oxygen-carrying protein | Hemoglobin | Hemocyanin |
| Metal in that protein | Iron | Copper |
| Color | Red | Blue or blue-green |
Does it have an alimentary canal?
Yes. Almost all animals except very simple ones such as sponges and cnidarians have a gastrointestinal tract.
How are invertebrates classified according to number of legs?
| Number of legs | Groups |
| 6 | Hexapoda (subphylum), Insecta (class) |
| 8 | Arachnida (class) – spiders, mites, ticks, scorpions Octopus |
| 10 | Decapoda (order) – crabs, lobsters, shrimp, crayfish Squids |
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