Zebrasoma flavescensPetromyzon marinusGaleocerdo cuvier Branchiostoma lanceolatumPyura spinifera

Fish were the first vertebrates on earth, having the oldest fossil known, dated about 550 million years ago. Fish are related to the evolution of all land vertebrates, such as reptiles, amphibians, etc. First invertebrates appeared on earth in the Proterozoic period, about 650 million years ago. From here, other animals evolved from invertebrates.

Fish have bilateral symmetry, which means if it were to be cut in half, the two parts would exactly be the same. In invertebrate chordates, urochordatas have radial symmetry, and cephalochordata have bilateral. Fish are coelomates, or animals with a true coelom. Invertebrate chordates, such as cephalochordatas are also coelomates. Fish are vertebrates, and have bones, such as back bones to support its body. Some are also made of cartilage. They provide protection and gives structure. They also have fins and a tail that extends from the anus. Invertebrate chordates, such as cephalochordatas have no backbone, but have a muscular notochord instead, and they are unlike vertebrates.

Some invertebrate chordates, such as tunicates require no special organs for excretion, but use ammonia which diffuses into the sea water. Others such as Lancelets have excretion organs such as protonephridia. Fish maintain balance by retaining high concentrations of urea and trimethyl amine oxide in their blood. The urea increases the salt concentration to the level of sea-water. To keep salt concentrations low similar to the environment, elasmobranchs secrete salt through the kidneys and a special gland, the rectal gland, which connects to the alimentary canal. The rectal gland concentrates and eliminates both salt and chloride ions from the body tissues.

A fish’s circulatory system carries blood from the heart through the gills and to the body tissues before returning to the heart. Its heart is a series of four chambers with deoxygenated blood running through the heart to the gills without returning to the heart, The heart chambers are separated by valves to prevent blood flowing in the wrong direction during ventricular pumping. In invertebrate chordates the blood is propelled by a distinct heart; Unoxygenated blood is propelled forward by a vessel called the ventral aorta. It then passes through a series of branchial arteries in the gills, where gas exchange takes place, and the oxygenated blood flows to the body, much of it returns to its origin via a dorsal aorta.

Invertebrate chordates- Cephalochordatas- Water is taken through its mouth by cilia, and then passes through the gill slits, which filters out food particles, while water is passed on. Digestive enzymes are released and digested in the iliocolonic ring. Urochordata- are mostly sessile, unable to move and attached to rocks, which filter out food by having many gill slits from the water like sponges. Fish can eat from algae, plants, to animals. Esophagus are used, and most fishes have stomachs. The pancreas excrete digestive enzymes and digest it. The intestines absorb the food nutrition, and wastes are excreted through the anus.

There is a wide range of reproduction for fish. Fish can be males, females, or hermaphrodites. Basic reproduction for hermaphrodites is eggs produced in the ovaries, and sperm produced in testes combine later when released into the body cavity. Most fish reproduce externally. Ovaries in the female produce eggs while Testes in the male produce sperm. The eggs and sperm are released through an opening behind the anus. Some bony fishes, called livebearers yield live babies; the male injects sperm into the female by using his anal fin. (internal fertilization) And then the female carriers the eggs until they are released. Most Invertebrate Chordates are hermaphroditic. They store there eggs inside their bodies until they hatch, while sperm is released into the water where it fertilizes other individuals in with incoming water.

Gills are the primary respiratory organ of fish. They are located lateral to the mouth cavity During ventilation water flows into the mouth across the gill and through the gill slits. Urochordata- Filters out food and oxygen from the water in gill slits Cephalochordata- Respires directly through the body walls.

Do sharks sleep? Fish don't sleep in the same way that we do, but they have active and inactive periods. Some sharks like the nurse shark have been observed resting motionless on the seafloor. Others have to keep moving in order to breathe. Many species of open ocean shark need to swim constantly to keep water flowing across their gills, and to keep themselves from sinking.