Fin

300px
Fins are used by aquatic animals, such as this orca, to generate thrust and control the subsequent motion
Caudal fin of a great white shark
Aquatic animals typically use fins for locomotion
(1) pectoral fins (paired), (2) pelvic fins (paired), (3) dorsal fin, (4) adipose fin, (5) anal fin, (6) caudal (tail) fin
Comparison between A) the swimming fin of a lobe-finned fish and B) the walking leg of a tetrapod. Bones considered to correspond with each other have the same color.
In a parallel but independent evolution, the ancient reptile Ichthyosaurus communis developed fins (or flippers) very similar to fish (or dolphins)
In the 1990s the CIA built a robotic catfish called Charlie to test the feasibility of unmanned underwater vehicles

Thin component or appendage attached to a larger body or structure.

- Fin
300px

16 related topics

Alpha

Pinniped

Restoration of Puijila
Fossil of Enaliarctos
Fossil skull cast of Piscophoca sp. from Phocidae
Reconstruction of Archaeodobenus akamatsui family Odobenidae
Male and female South American sea lions, showing sexual dimorphism
Light reflection on an elephant seal eye
Frontal view of brown fur seal head
Vibrissae of walrus
Weddell seal underwater
Northern elephant seal resting in water
Walrus on ice off Alaska. This species has a discontinuous distribution around the Arctic Circle.
Harbor seal hauled out on rock
Steller sea lion with white sturgeon
Leopard seal capturing emperor penguin
Orca hunting a Weddell seal
Walrus herd on ice floe
Northern fur seal breeding colony
Male northern elephant seals fighting for dominance and females
Harp seal mother nursing pup
Adult Antarctic fur seal with pups
Walrus males are known to use vocalizations to attract mates.
Sea lion balancing a ball
Inuit seal sculptures at the Linden Museum
Captive sea lion at Kobe Oji Zoo Kobe, Japan
Men killing northern fur seals on Saint Paul Island, Alaska, in the mid-1890s
Protests of Canada's seal hunts
Grey seal on beach occupied by humans near Niechorze, Poland. Pinnipeds and humans may compete for space and resources.

Pinnipeds (pronounced ), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals in the clade Pinnipedia.

Cetacea

Infraorder of aquatic mammals that includes whales, dolphins, and porpoises.

Infraorder of aquatic mammals that includes whales, dolphins, and porpoises.

Dolphin anatomy
Humpback whale fluke
Biosonar
Bubble net feeding
Killer whale porpoising
Two views of the skeletons of Dorudon atrox, extinct for 40 million years, and Maiacetus inuus, extinct for 47.5 million years, in the swimming position for comparison.
Cetaceans display convergent evolution with fish and aquatic reptiles
Fossil of a Maiacetus (red, beige skull) with fetus (blue, red teeth) shortly before the end of gestation
Whales caught 2010–2014, by country
Dominoes made of baleen
A whale as depicted by Conrad Gesner, 1587, in Historiae animalium
"Destruction of Leviathan" engraving by Gustave Doré, 1865
Silver coin with Tarus riding a dolphin
Constellation Cetus
Depiction of baleen whaling, 1840
Stranded sperm whale engraving, 1598
Sea World show featuring bottlenose dolphins and false killer whales
Ulises the orca, 2009
Dawn Brancheau doing a show four years before the incident
SeaWorld pilot whale with trainers

The fluke is set horizontally on the body, unlike fish, which have vertical tails.

Jellyfish in motion

Aquatic locomotion

Biologically propelled motion through a liquid medium.

Biologically propelled motion through a liquid medium.

Jellyfish in motion
A eurasian coot swimming
The flagellum of a Gram-negative bacteria is rotated by a molecular motor at its base
Salmon spermatozoa for artificial propagation
Shrimp paddle with special swimming legs (pleopods)
Daphnia swims by beating its antennae
Octopuses swim headfirst, with arms trailing behind
Jellyfish pulsate their bell for a type of jet locomotion
Scallops swim by clapping their two shells open and closed
Open water fish, like this Atlantic bluefin tuna, are usually streamlined for straightline speed, with a deeply forked tail and a smooth body shaped like a spindle tapered at both ends.
Many reef fish, like this queen angelfish, have a body flattened like a pancake, with pectoral and pelvic fins that act with the flattened body to maximize manoeuvrability.
The leopard shark angles its pectoral fins so they behave as hydrofoils to control the animal's pitch
The slowest-moving fishes are the sea horses, often found in reefs
Common toad (Bufo bufo) swimming
Nile crocodile (Crocodylus niloticus) swimming
Immature Hawaiian green sea turtle in shallow waters
Macroplata
Comparative skeletal anatomy of a typical otariid seal and a typical phocid seal
Animated representation of lobstering
Chinstrap penguin leaping over water
Swimming dog

Hydrofoils, or fins, are used to push against the water to create a normal force to provide thrust, propelling the animal through water.

The Soyuz TMA-9 spacecraft launches from the Baikonur Cosmodrome, Site 1/5 in Kazakhstan

Rocket

Spacecraft, aircraft, vehicle or projectile that obtains thrust from a rocket engine.

Spacecraft, aircraft, vehicle or projectile that obtains thrust from a rocket engine.

The Soyuz TMA-9 spacecraft launches from the Baikonur Cosmodrome, Site 1/5 in Kazakhstan
Depiction of rocket arrows, from the Huolongjing. The left arrow reads 'fire arrow' (huo jian), the middle is an 'dragon shaped arrow frame' (long xing jian jia), and the left is a 'complete fire arrow' (huo jian quan shi)
An East India Company battalion was defeated during the Battle of Guntur, by the forces of Hyder Ali, who effectively utilized Mysorean rockets and rocket artillery.
William Congreve at the bombardment of Copenhagen (1807) during the Napoleonic Wars.
Goddard with a liquid oxygen-gasoline rocket (1926)
A V-2 rocket launched from Test Stand VII in summer 1943
Viking 5C rocket engine
Gas Core light bulb
Illustration of the pendulum rocket fallacy. Whether the motor is mounted at the bottom (left) or top (right) of the vehicle, the thrust vector (T) points along an axis that is fixed to the vehicle (top), rather than pointing vertically (bottom) independent of vehicle attitude, which would lead the vehicle to rotate.
A Trident II missile launched from sea.
A Bumper sounding rocket
Apollo LES pad abort test with boilerplate crew module.
Doglegged flight path of a PSLV launch to polar inclinations avoiding landmass.
Workers and media witness the Sound Suppression Water System test at Launch Pad 39A.
A balloon with a tapering nozzle. In this case, the nozzle itself does not push the balloon but is pulled by it. A convergent/divergent nozzle would be better.
Rocket thrust is caused by pressures acting on both the combustion chamber and nozzle
Forces on a rocket in flight
A map of approximate Delta-v's around the Solar System between Earth and Mars
The Tsiolkovsky rocket equation gives a relationship between the mass ratio and the final velocity in multiples of the exhaust speed
Spacecraft staging involves dropping off unnecessary parts of the rocket to reduce mass.
Apollo 6 while dropping the interstage ring
during launch phase
torn apart T+73 seconds after hot gases escaped the SRBs, causing the breakup of the Shuttle stack
A battery of Soviet Katyusha rocket launchers fires at German forces during the Battle of Stalingrad, 6 October 1942

They may also have one or more rocket engines, directional stabilization device(s) (such as fins, vernier engines or engine gimbals for thrust vectoring, gyroscopes) and a structure (typically monocoque) to hold these components together.

Tetrapod

Tetrapods are four-limbed animals constituting the superclass Tetrapoda.

Tetrapods are four-limbed animals constituting the superclass Tetrapoda.

Carl Linnaeus's 1735 classification of animals, with tetrapods occupying the first three classes
Devonian fishes, including an early shark Cladoselache, Eusthenopteron and other lobe-finned fishes, and the placoderm Bothriolepis (Joseph Smit, 1905).
Fossil of Tiktaalik
Eusthenopteron, ≈385 Ma
Tiktaalik, ≈375 Ma
Acanthostega, ≈365 Ma
Ichthyostega, 374–359 Ma
Edops, 323-299 Ma
Diadectes, 290–272 Ma
Cross-section of a labyrinthodont tooth

The tetrapod's ancestral fish, tetrapodomorph, possessed similar traits to those inherited by the early tetrapods, including internal nostrils and a large fleshy fin built on bones that could give rise to the tetrapod limb.

Fish

Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits.

Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits.

Dunkleosteus was a gigantic, 10 m long prehistoric fish of class Placodermi.
Lower jaw of the placoderm Eastmanosteus pustulosus, showing the shearing structures ("teeth") on its oral surface; from the Devonian of Wisconsin
Leedsichthys, of the subclass Actinopterygii, is the largest known fish, with estimates in 2005 putting its maximum size at 16 m.
A relative of the seahorses, the leafy seadragon's appendages allow it to camouflage (in the form of crypsis) with the surrounding seaweed.
The psychedelic mandarin dragonet is one of only two animal species known to have blue colouring because of cellular pigment.
Diversity of various groups of fish (and other vertebrates) through time
Lungfish are the closest living relatives of tetrapods (four-limbed vertebrates).
The bowfin Amia calva is the sole survivor of the halecomorph clade.
Organs: 1. Liver, 2. Gas bladder, 3. Roe, 4. Pyloric caeca, 5. Stomach, 6. Intestine
Tuna gills inside the head. The fish head is oriented snout-downwards, with the view looking towards the mouth.
Didactic model of a fish heart
Dorsal view of the brain of the rainbow trout
The anatomy of Lampanyctodes hectoris (1) operculum (gill cover), (2) lateral line, (3) dorsal fin, (4) fat fin, (5) caudal peduncle, (6) caudal fin, (7) anal fin, (8) photophores, (9) pelvic fins (paired), (10) pectoral fins (paired)
Swim bladder of a rudd (Scardinius erythrophthalmus)
Ovary of fish (Corumbatá)
French grunts – Haemulon flavolineatum
Whale sharks, the largest species of fish, are classified as vulnerable.
These fish-farming ponds were created as a cooperative project in a rural village.
Fish counter display at the Oulu Market Hall in Oulu, Finland.
A Bengali fish vendor
Avatar of Vishnu as a Matsya
The ichthus is a Christian symbol of a fish signifying that the person who uses it is a Christian.
These goldband fusiliers are schooling because their swimming is synchronised.
Agnatha (Pacific hagfish)
Chondrichthyes (Horn shark)
Actinopterygii (Brown trout)
Sarcopterygii (Coelacanth)
Egg of lamprey
Egg of catshark (mermaids' purse)
Egg of bullhead shark
Egg of chimaera

In biology, the term fish is most strictly used to describe any animal with a backbone, gills throughout life, and limbs (if any) in the shape of fins. Many types of aquatic animals with common names ending in "fish" are not fish in this sense; examples include shellfish, cuttlefish, starfish, crayfish and jellyfish. In earlier times, even biologists did not make a distinction – sixteenth century natural historians classified also seals, whales, amphibians, crocodiles, even hippopotamuses, as well as a host of aquatic invertebrates, as fish.