Agnatha
The class Agnatha includes organisms such as the extinct fishlike animals called ostracoderms and the modern-day sea lamprey and composes some of the oldest vertebrate fossils. Only about 60 species of Agnathans exist today as most of these groups disappeared during the Devonian period. They were the earliest fish inthe fossil record. The word agnatha means "without a jaw". The early jawless fish used their gill slits to filter food as well as to exchange gas. There are only a few species of agnatha left today, they are the Lamprey and the Hagfish.
Characteristics
This distinct class consists of jawless creatures, which are generally small in length. Most are bottom-dwellers, and lacking fins, wriggle along the sea floor or streambeds. However, there are some Agnathans that do have fins and are quite active. The mouths of this class are circular or slit-like openings.
Lampreys are parasitic and feed on the body fluids of their host. The Lampreys obtain these body fluids by attaching themselves onto the body of the host fish, cutting a hole in its flesh, using their tongue, and then feeding on the blood and tissue inside. This is called suspension feeding. The agnatha have a closed circulatory system with three types of blood vessels. Arteries move blood away from the heart, veins move blood back to the heart, and capillaries (which are thin enough to allow oxygen and nutrients to diffuse into the body cells). The Lamprey has several primitive features; The skeleton is very simple, the notochord remains during adulthood, the skull is made on cartilage, they lack paired fins, and they have multiple gill slits. Unlike the Lamprey, the Hagfish is not a parasite, but a scavenger. (Feeds on sick or dead fish)
Some Agnathans live as larvae in freshwater streams, attaining sexual maturity when they reach salt water. Others completely skip the larval stage and live in saltwater their entire lives.
 |
 |
Digestive System
Cirulatory System
Excretory System
Respiratory System
