An octopus has a round head and a mantle along with eight arms. The arms are endowed with one or two rows of suckers but these never have hooks or sucker rings. Squids are also cephalopods with a triangular shaped head, a mantle and eight arms. Along with that they have two fins on their head and two tentacles. The arms of a Squid are endowed with hooks and/or suckers or sucker rings. The tentacles are arranged in pairs.
edit Anatomical Differences
Octopi have almost entirely soft bodies with no internal skeleton. They have neither a protective outer shell like the nautilus, nor any vestige of an internal shell or bones, like cuttlefish or squids. A beak, similar in shape to a parrot's beak, is the only hard part of their body. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from morays or other predatory fish.
In a squid, the main body mass is enclosed in the mantle, which has two swimming fins along each side. It should be noted that these fins, unlike in other marine organisms, are not the main source of ambulation in most species. The skin of the squid is covered in chromatophores, which enable the squid to change color to suit its surroundings. The underside of the squid is also found to be lighter than the topside, in order to provide camouflage from both prey and predator.
Both octopi and squid have three hearts. Two pump blood through each of the two gills, while the third pumps blood through the body. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen. The hemocyanin is dissolved in the plasma instead of being bound in red blood cells and gives the blood a blue color. Octopi draw water into their mantle cavity where it passes through its gills. As mollusks, octopuses have gills that are finely divided and vascularized outgrowths of either the outer or the inner body surface. The hearts of a squid are surrounded by the renal sacs - the main excretory system of the squid. The kidneys are faint and difficult to identify and stretch from the hearts (located at the posterior side of the ink sac) to the liver.
edit Difference in Size
The majority of squid are no more than 60 cm long, although the giant squid may reach 13 m in length. There have even been claims reported of specimens of up to 20 metres (66 ft).
Octopuses grow in size from upto 1cm to about 5m. They have a short life span and may not live upto more than 4 to 5 years but can grow quite heavy in weight.
Octopuses and squids move by "jet propulsion", sucking water into a muscular sac in the mantle cavity surrounding their bodies and quickly expelling it out a narrow siphon. Octopuses and squids can swim in any direction and can alter their course quickly. Squids use fins located on their heads to propel themselves when swimming at low speeds. These fins steer and stabilize the squids when moving slowly, and wrap around the body when they move quickly, by way of jet propulsion. Most octopuses do not have fins as adults. Some deepwater octopuses are exceptions. The eyes of a Squid, found on either side of the head, each contain a hard lens. The lens is focused by moving, much like the lens of a camera or telescope, rather than changing shape like a human eye.
edit Finding their prey
Octopuses use their eight sucker-lined arms to capture their prey and move about on the ocean floor. Squids have eight arms lined with suckers and two specialized tentacles that they use to reach out and capture prey. Octopuses pierce the shells of their prey, injecting venom that causes paralysis. They then release salivary enzymes, loosening the meat from the inner shell. Squids use their two specialized tentacles to quickly reach out and capture fishes. They tear off bits of flesh and scrape the meat into their mouths with their beaks.
The male octopus uses a specialized arm called a hectocotylus to transfer sperm to the mantle cavity of a receptive female. The female lays strings of fertilized eggs on the roof of her den. She guards, cleans and aerates the eggs with water expelled from her siphon until hatching—anywhere from 30 days to a year, depending on the species. The female may build a wall of rocks to seal off the den and will remain in the den until just before she dies, after the eggs have hatched. Squids often mate in large groups, and attach their egg capsules to the ocean floor or to seaweed. Most adult octopuses and squid die after reproducing. Their bodies are recycled in the food web, nourishing other animals, and ultimately providing food for their young when they hatch.
edit Geographical distribution
Squids and Octopuses are found in salty water from the tropics to the temperate zones.
edit As Food
On menus, squid may be called calamari. Black pasta is colored using squid ink. The arms, tentacles and ink are also edible; in fact, the only part of the squid that is not eaten is its beak and gladius (pen). Squid rings and arms are often coated in batter and fried in oil. In the Mediterranean, squid or cuttlefish ink is eaten in a variety of dishes such as paella, risotto, soups and pasta. In Chinese and South East Asian cuisine, squid is a common ingredient in a variety of dishes such as stir-fries, rice and noodle dishes.
Many species of octopus are eaten as food by human cultures around the world. Greek restaurants sometimes serve pickled octopus, called octopothi. Octopus is a common ingredient in Japanese cuisine, including sushi, takoyaki, and Akashiyaki. Some small species are sometimes eaten alive as a novelty and health food (mostly in South Korea). Cooked octopus contains approximately 139 calories per three ounce portion, and is a source of vitamin B3, B12, potassium, phosphorus, and selenium.
Fishers trawl for octopuses using weighted chains that drag along the ocean floor, scaring the octopuses into a net. Another method involves lowering traps and pots which octopuses will use as shelters. Spear fishing and drift fishing are also practiced. Fishers catch squids by jigging. They shine bright lights and drop lines into the water with special lures called jigs, which they jerk up and down, attracting squids to the light and movement. Recently, fishers have begun to use large seine nets that encircle the squids, forming pockets and trapping them.