Cyclostomi

Some fish have round mouths. They do not have jaws. These fish are called lampreys and hagfish.

Hagfish and lampreys are very old. They have small parts that work like teeth. These parts help them eat.

Lampreys start as tiny larvae. They live in fresh water. Later, they change into adults.

Hagfish live in the salty sea. They have a special mouth. It works like a moving belt.

These fish are very different from other fish. It is fun to learn about them!

Some fish are very special. They do not have jaws. These fish are called cyclostomes. This name means "circular mouth."

Cyclostomes include two groups. They are lampreys and hagfishes. Both groups have mouths with hard parts. These parts work like teeth.

Lampreys and hagfishes are quite different. Lampreys start life as tiny larvae. They live in fresh water. They go through a change called metamorphosis. This turns them into adults.

Hagfishes live in the salty sea. They have a unique mouth. It has plates that move like a conveyor belt. This helps them eat.

These fish also have different bodies. Lampreys have two parts in their inner ear. Hagfishes have only one. Lampreys have a special organ to help them eat. It is called an endostyle. This organ later turns into a thyroid gland. This gland helps the body work. Scientists study these fish to learn about history. They help us see how early life began.

Cyclostomes are a special group of vertebrates. Their name comes from Greek words meaning "circular mouth." This group includes two types of jawless fish. These are the lampreys and the hagfishes. They are very different from most fish we see today. Most fish have jaws to help them eat. Cyclostomes do not have these bony jaws. Instead, they have hard parts in their mouths. These parts are called ceratodontes. They work much like teeth to help the fish feed.

These fish have very unique ways of working. In a hagfish, the mouth has a special plate. This plate has grooves on its floor. The tooth plates slide over these grooves. They move back and forth like a conveyor belt. Lampreys have a different way of eating. They have tooth plates on a tongue-like structure. Lamprey larvae also have a special organ called an endostyle. This organ helps them filter food from the water. Later, the endostyle turns into a thyroid gland. This is a big change in their bodies.

Scientists have spent a long time studying these fish. They wanted to know how they are related. For a long time, people disagreed on this. The "vertebrate hypothesis" said lampreys were closer to jawed fish. The "cyclostome hypothesis" said lampreys and hagfishes were close relatives. Most studies of body parts supported the first idea. However, most studies of DNA supported the second idea. Recent studies of how they grow suggest they are more similar. This helps support the idea that they are close relatives.

There are many interesting facts about their bodies. Hagfishes and lampreys both have only one gonad. In hagfishes, the left one simply disappears as they grow. In lampreys, the left and right ones fuse together. Their ears are also quite different from ours. Most vertebrates have three tiny canals in their inner ear. Lampreys have only two of these canals. Hagfishes have just one single canal. Even their nostrils are simple. They each have only one nostril. In lampreys, the nasal duct ends in a pouch. In hagfishes, it opens into the throat.

Learning about cyclostomes helps us understand the history of life. They show us how early vertebrates might have lived. Some very old jawless fish lived 500 million years ago. These fossils show that some early fish had hard coverings. We think the common ancestor of all vertebrates had mineralized tissues. Hagfishes and lampreys likely lost these hard parts over time. By studying these living fish, we see a window into the past. They connect us to the very first animals with backbones.

Cyclostomi is a superclass of vertebrates that includes two living classes of jawless fish. These animals are the lampreys and the hagfishes. The name Cyclostomi comes from Ancient Greek words meaning "circular mouth." This name describes their unique anatomy. Unlike most vertebrates, these fish lack bony jaws to grasp food. Instead, they possess horny epidermal structures called ceratodontes. These structures function much like teeth. They also have branchial arches, which are structures used for breathing. In cyclostomes, these arches are positioned internally. This differs from the external positioning found in related jawed fishes.

The feeding mechanisms of these two groups are quite distinct. Lampreys possess tooth plates located on a tongue-like piston cartilage. This allows them to feed in a specific way. Hagfishes use a different method involving a fixed cartilaginous plate on the floor of the mouth. This plate contains grooves. These grooves allow tooth plates to slide back and forth like a conveyor belt. As the plates move over the edge of the plate, they are everted. Hagfishes also have a keratinous palatine tooth that hangs from the roof of their mouths.

Both lampreys and hagfishes have unique internal biological processes. For example, they both possess only a single gonad. However, the reasons for this are different. In hagfishes, the left gonad simply degenerates during ontogeny, which is the process of development. Only the right gonad actually develops. In lampreys, the left and right gonads fuse together into one. Neither species has gonoducts. Their development also follows different paths. Hagfishes undergo direct development. Lampreys, however, go through a larval stage called ammocoetes. These larvae live in freshwater. During metamorphosis, the lamprey endostyle, an organ used for filter feeding, develops into the thyroid gland.

Scientists have long debated the evolutionary history of these animals. This debate centers on whether Cyclostomi is a monophyletic group. A monophyletic group includes a common ancestor and all its descendants. The "vertebrate hypothesis" suggested that lampreys are more closely related to jawed vertebrates, known as gnathostomes, than to hagfishes. Conversely, the "cyclostome hypothesis" suggests lampreys and hagfishes are close relatives. Most studies of anatomy supported the vertebrate hypothesis. However, most molecular phylogenies, which study DNA, support the cyclostome hypothesis. Recent evidence regarding embryonic development may also support the cyclostome hypothesis.

There are many significant physiological differences between cyclostomes and other vertebrates. For instance, they lack several common organs. They do not have a thymus, a spleen, myelin, or sympathetic chain ganglia. Their sensory systems are also simplified. Both groups have only a single olfactory organ and a single nostril. In lampreys, the nasal duct ends blindly in a pouch. In hagfishes, the duct opens into the pharynx. Their inner ears are also different. Most vertebrates have three semicircular canals in the inner ear. Lampreys have only two. Hagfishes have only one. It is assumed the hagfish canal resulted from two canals merging.

Understanding cyclostomes provides a window into the history of vertebrate life. Some of the oldest fish in the world lived 500 million years ago. Fossil evidence from groups like the ostracoderms shows that many ancient jawless fish had armored bodies. For example, anaspids had armored body coverings. Other species like Lasanius had scutes, which are small bony plates. The presence of these mineralized elements suggests a common ancestor. This ancestor likely had mineralized tissues. It is believed that hagfish and lampreys lost these hard tissues over time. This connection helps scientists trace the evolution of all vertebrates.