Short answers, pulled from the story.
The Hedgehog was a forward-throwing anti-submarine mortar developed by the Royal Navy and deployed on convoy escort ships such as destroyers and corvettes to attack U-boats. It fired a salvo of 24 spigot mortar bombs in a circular pattern roughly 180 metres ahead of the ship. The weapon entered service in 1942.
The Hedgehog's contact fuzes detonated only on direct impact with a submarine's hull, bypassing the need to set a correct depth before firing. A depth charge relied on blast pressure transmitted through a cushion of water, which absorbed much of the force; a Hedgehog hit struck the hull directly. By the end of World War II the weapon achieved a kill in roughly one in five attacks, compared to fewer than one in eighty for depth charges.
The Hedgehog entered service in 1942. Its prototype launcher was tested aboard ship in 1941, but no submarine kills were recorded until November 1942, by which point the weapon had been installed on one hundred ships.
The Hedgehog was developed by the Directorate of Miscellaneous Weapons Development, adapting the spigot mortar design created by Lieutenant Colonel Stewart Blacker of the Royal Artillery. Major Millis Jefferis of MIR(c) adapted Blacker's design for naval use. The weapon replaced the unsuccessful Fairlie Mortar, which had been secretly tested aboard ship in 1941.
Three 'Hedgerow' flotillas of Landing Craft Assault boats carried the Hedgehog to the Normandy landings in place of troops. The projectiles were fitted with nose extensions to detonate above ground, clearing 100-yard-wide paths through mines and barbed wire on the beach.
The Royal Navy introduced the Squid in late 1943 and the three-barrelled Limbo, firing 400 lb Minol charges, in 1955. The United States developed Mousetrap and then Weapon Alpha before replacing both with ASROC. The Soviet Union built its own copy in 1949, the MBU-200, followed by the MBU-600 in 1956 with a range of 644 metres.