Ascidiacea

Sea squirts live in the ocean.

They have a tough skin. It acts like a little house. Most sea squirts stay in one place. They stick to rocks or shells.

They eat by pulling in water. The water goes through a tube. This helps them find food to eat. They can squirt water out fast. This is how they got their name. Some sea squirts live in big groups. They can live for many years.

They are very cool animals to see!

Sea squirts are small animals that live in the ocean.

They are often called ascidians. Most sea squirts stay in one place. They stick to rocks or shells. They have a tough outer layer called a tunic. This tunic is made of cellulose. Cellulose is the same stuff found in plants.

There are 2,300 different species of sea squirts. Some live all by themselves. These are solitary ascidians. Others live in big groups. These are colonial ascidians. In a colony, many small parts called zooids live together.

Sea squirts eat by filtering water. They pull water in through a tube called an oral siphon. This tube acts like a mouth. Inside, tiny hairs called cilia move the water. The water flows through slits in the throat. A sticky mucus traps food in the water. The food then moves into the stomach. To get rid of waste, they push water out through a second tube. This is called an atrial siphon. If you touch them, they may squirt water out very fast!

Sea squirts are fascinating marine animals known as ascidians.

They belong to a group called Tunicata. Most sea squirts live in shallow ocean water. They prefer places where the salt level is over 2.5%. These animals are mostly sessile. This means they stay attached to one spot. They use a hard outer layer called a tunic to protect themselves. This tunic is made of cellulose. Cellulose is a material usually found in plants.

Feeding is a very organized process for a sea squirt. It begins at the oral siphon, which acts like a mouth. Tiny hairs called cilia beat to pull water inside. The water moves through a part called the pharynx. This area has many small holes called stigmata. An endostyle produces a sheet of sticky mucus. This mucus traps tiny food particles as the water passes through. The food then moves down into the stomach for digestion. Finally, the water exits through a second tube called the atrial siphon.

Scientists have studied these creatures to understand how life changes. Many sea squirts go through a process called retrogressive metamorphosis. This sounds strange because it means they become simpler as adults. They start life as swimming larvae. These larvae have a tail and a structure called a notochord. A notochord is a rod of exactly 40 cells. When the larva finds a rock, it settles down. It then loses its tail and many complex parts. The animal rearranges itself into the adult form.

There are about 2,300 different species of ascidians in the world. They can be solitary or live in large groups. Solitary ascidians live alone as single animals. Colonial ascidians live in big communities. In these colonies, many small individuals called zooids live together. Sometimes, these zooids even share the same blood system. Some colonial species can live for many decades.

Some species even have blood that looks green or red. This color comes from special cells in their blood.

Even though they look simple, their bodies are very active. A sea squirt has a heart that is a muscular tube. This heart does something very unusual. It changes the direction it pumps blood every three to four minutes. This helps move blood through different vessels in the body. You can see how they relate to other animals by looking at their nerves. Their nervous system starts as a tube. This is very similar to the spinal cords found in vertebrates.

Ascidians, which are commonly called sea squirts, are a diverse class of marine invertebrates.

They belong to the subphylum Tunicata and are known as sac-like filter feeders. Most species live in shallow marine environments where the salinity is higher than 2.5%. While some relatives like salps swim freely in the water, most ascidians are sessile. This means they live attached to a solid surface like a rock or a shell. They are protected by a tough outer covering called a tunic. This tunic is unique because it is made of cellulose, a polysaccharide typically found in plants.

Unlike the hard shells of molluscs, the tunic is living tissue that often has its own blood supply.

The feeding process in a sea squirt is a highly organized mechanical system. It begins when water is drawn into the oral siphon, which acts as the mouth. Inside, tiny hair-like structures called cilia beat to pull the water into the pharynx. The pharynx contains many small perforations known as pharyngeal stigmata. To catch food, an organ called the endostyle produces a sheet of mucus. This mucus flows across the pharynx and traps planktonic food particles as they pass through the stigmata. The food is then collected by a dorsal ridge and moved into the esophagus. From there, it travels to the stomach in the abdomen for digestion. The water finally exits the body through a second opening called the atrial siphon.

Ascidians can be categorized into three distinct biological types based on their social structures. Solitary ascidians live as completely independent individuals. Social ascidians form clumped communities by attaching themselves at their bases. The third type is the colonial ascidian, which consists of many small individuals called zooids. These zooids live together to form large, complex colonies. In some highly developed colonial species, the individuals are so closely linked that they share a single cloaca. This means all their atrial siphons open into one common chamber. Some of these colonies are incredibly long-lived and can survive for many decades.

The circulatory system of a sea squirt contains several remarkable biological features. The heart is a curved, muscular tube located near the stomach. One of the most unusual facts about this heart is its rhythm. The heart alternates the direction in which it pumps blood every three to four minutes. It uses two excitatory areas to switch the flow between different vessels. The blood contains different types of cells, including lymphocytes and phagocytic amoebocytes. Some species even have colorful blood due to special cells called vanadocytes. These cells contain pigments like hemoglobin, which makes the blood red, or vanadium, which makes it green.

The life cycle of an ascidian involves a process called retrogressive metamorphosis.

Most ascidians are hermaphrodites, meaning they possess both male and female reproductive organs. They begin life as free-swimming, tadpole-like larvae. These larvae are quite advanced, possessing a long tail, muscles, and a notochord. The notochord is a structural rod made of exactly 40 cells. When the larva finds a suitable surface, it uses adhesive secretions to attach itself. Once attached, the animal undergoes a massive transformation where it loses its tail and complex larval organs. It rearranges its remaining parts to become a simple, sessile adult.

Scientists find the anatomy of ascidians very important for understanding evolutionary history. The larval stage shows clear affinities to chordates, which is the group containing vertebrates. The larva has a hollow dorsal nerve tube that is very similar to the spinal cords of more complex animals. Even though the adult lacks a true brain, it has a large ganglion between the siphons. This nervous system structure helps scientists hypothesize that true chordates may have evolved from sexually mature larvae. This connection between the simple adult and the complex larva is a landmark in biological theory.

There are approximately 2,300 known species of ascidians found throughout the world's oceans.

They occupy various niches, from solitary individuals to massive colonial structures. These colonies can be made of genetically identical zooids that function as a single unit. Some species, such as those in the family Molgulidae, have even evolved to skip the swimming larval stage entirely through direct development. Whether they are colorful individuals like Polycarpa aurata or vast colonies like Didemnum molle, ascidians represent a unique way of life in the sea.

Their ability to turn plant-like cellulose into a protective home makes them one of nature's most interesting experiments.