Pisces — built for water
This is the class of fishes, well adapted to living in water. They live in sea water and fresh water environments.
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- They bear a bony or cartilaginous internal skeleton.
- The body is streamlined so as to move easily through water.
- The body is covered with scales.
- They bear fins, for swimming and for balance.
- They have a lateral line system able to detect vibrations in the water.
- The heart has two chambers — a single atrium and a single ventricle.
- Respiration is carried out by gills.
- They are poikilothermic — body temperature changes with the temperature of the environment.
- They have eyes without eyelids.
Two classes of fish
Fishes are separated into two classes on the basis of how the internal skeleton is made.
| Chondrichthyes | Osteichthyes |
|---|---|
| The internal skeleton is cartilage. | The internal skeleton is bone. |
| Live in sea water only. | Live in both sea water and fresh water. |
| About 10% of fishes. | About 90% of fishes. |
| Gill slits are not covered by an operculum. | Gill slits are covered by a pair of opercula. |
| The mouth is anterior and ventral. | The mouth is at the anterior end. |
| Examples — shark, ray | Examples — tuna, tilapia, seahorse |
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