no, crabs are not migrated spiders.. crabs are crustaceans..spiders are arachnids..there is little connection

None

I wouldn't think so. Weren't horseshoe crabs there before the dinosaurs?

No. It is true that dolphins are essentially dogs (Pakicetus) that migrated to the sea but it isn't the case with crabs and spiders. They have quite a few similar characteristics, i.e. the exoskeleton, 8 legs, jointd legs, a pair of appendages are not legs and they are both carnivores. They have quite a few differences aswell though, i.e. the exoskeleton is a lot harder in crabs, the legs are used mainly for walking for spiders but for crabs they have many uses, spiders have 2 body parts whereas crabs have 3 and spiders are usually nocturnal. All in all I would say that arachnids and crustaceans are siblings if you will in the evolutionary chain, like birds and mammals, but not one is descended from the other.

The phylum of arthropods -- a top-level category within the animal kingdom -- includes crustaceans, arachnids (spiders), insects, and other creatures with exoskeletons and jointed appendages. They all first evolved in the sea. Land animals capable of breathing air came later. The common ancestor of spiders and crabs was a marine arthropod.

No, both are arthropods and their evolution started in the sea, at the Cambrian era. 1) "Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (Greek: brachy = short, ura = tail), or where the reduced abdomen is entirely hidden under the thorax. They are generally covered with a thick exoskeleton, and are armed with a single pair of chelae (claws). Crabs are found in all of the world's oceans; there are also many freshwater and terrestrial crabs, particularly in tropical regions. Crabs vary in size from the pea crab, only a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 m." "Kingdom: Animalia Phylum: Arthropoda Subphylum: Crustacea Class: Malacostraca Order: Decapoda Suborder: Pleocyemata Infraorder: Brachyura" Source and further information: http://en.wikipedia.org/wiki/Crab 2) "Spiders are predatory invertebrate animals that have two body segments, eight legs, no chewing mouth parts and no wings. They are classified in the order Araneae, one of several orders within the larger class of arachnids, a group which also contains scorpions, whip scorpions, mites, ticks, and opiliones (harvestmen). The study of spiders is known as arachnology. All spiders produce silk, a thin, strong protein strand extruded by the spider from spinnerets most commonly found on the end of the abdomen. Many species use it to trap insects in webs, although there are also many species that hunt freely. Silk can be used to aid in climbing, form smooth walls for burrows, build egg sacs, wrap prey, and temporarily hold sperm, among other applications. All spiders except those in the families Uloboridae and Holarchaeidae, and in the suborder Mesothelae (together about 350 species) can inject venom to protect themselves or to kill and liquefy prey. Only about 200 species, however, have bites that can pose health problems to humans. Many larger species' bites may be quite painful, but will not produce lasting health concerns. Spiders are found all over the world, from the tropics to the Arctic, living underwater in silken domes they supply with air, and on the tops of mountains. In 1973 Skylab 3 took two spiders into space to test their web-spinning capability in free-fall." "Kingdom: Animalia Phylum: Arthropoda Class: Arachnida Order: Araneae" Source and further information: http://en.wikipedia.org/wiki/Spider 3) Not exactly crabs, the anomura: "Anomura (sometimes Anomala) are a group of decapod crustaceans, including hermit crabs and others. Although the names of many anomurans includes the word crab, all true crabs are in the sister group to the Anomura, the Brachyura (the two groups together form the clade Meiura). The name Anomala reflects the unusual variety of forms in this group; whereas all crabs share some obvious similarities, the various groups of anomurans are quite dissimilar. The name Anomura derives from an old classification in which reptant decapods were divided into Macrura (long-tailed), Brachyura (short-tailed) and Anomura (differently-tailed). As decapods (meaning ten-legged), Anomurans have ten pereiopods, but the last pair of these is often hidden inside the gill chamber (under the carapace) and is used for keeping the gills clean. Since this arrangement is never found in true crabs, a "crab" with only eight pereiopods must be an Anomuran." Source and further information: http://en.wikipedia.org/wiki/Anomura 4) Crabs and Anomura are decapods (10 legs), although the last 2 legs of the anomura are often hidden. Spiders have 8 legs. Some crabs are called spider crabs, because they look like spiders: http://en.wikipedia.org/wiki/Spider_crab Some spiders are called crab spiders, because they look like crabs: http://en.wikipedia.org/wiki/Crab_spider 5) Crabs and spiders belong to the arthropods: "Arthropods (Phylum Arthropoda, from Greek ἄρθρον arthron, "joint", and ποδÏŒς podos, "foot") are the largest phylum of animals and include the insects, arachnids, crustaceans, and others. Arthropods are characterised by the possession of a segmented body with appendages on at least one segment. They have a dorsal heart and a ventral nervous system. All arthropods are covered by a hard exoskeleton made of chitin, a polysaccharide, which provides physical protection and resistance to desiccation. Periodically, an arthropod sheds this covering when it moults. More than 80% of described living animal species are arthropods, with over a million modern species described and a fossil record reaching back to the late proterozoic era. Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms. They range in size from microscopic plankton (~¼ mm) up to forms several metres long. The largest living arthropod is the Japanese spider crab, with a leg span up to 3½ m (12 ft), and some prehistoric arthropods were even larger, such as Jaekelopterus and Arthropleura." "Arthropods are today almost universally considered to be monophyletic, i.e. they only arose once, a view supported by both morphological and molecular studies. Such a view contradicts the widespread view in the 1970s that the arthropods had evolved on several occasions from soft-bodied, annelid-like ancestors." Source and further information: http://en.wikipedia.org/wiki/Arthropoda "The Subphylum Chelicerata constitutes one of the major subdivisions of the Phylum Arthropoda, including the arachnids, horseshoe crabs, and related forms." Source: http://en.wikipedia.org/wiki/Chelicerata Further information: http://en.wikipedia.org/wiki/Arachnid http://en.wikipedia.org/wiki/Crustacean 6) "From about 570 to 530 million years ago, an evolutionary burst of life forms occurred, often referred to as the "Cambrian Explosion." This marks an important point in the history of life on earth, as most of the major lineages of animals got their starts during the Cambrian Period and have been evolving ever since. If we wound the clock back a little more than half a billion years to the Cambrian, we would find that life then was different from life today: - All life was aquatic. - Most life was relatively small. - Many animals had unusual body layouts. Many Cambrian animals seem bizarre at first glance, but are actually members of groups that are still around today — such as the arthropods." http://evolution.berkeley.edu/evolibrary/article/0_0_0/cambrian_02 "Sanctacaris was probably a chelicerate — the arthropod group that includes spiders, scorpions, and horseshoe crabs. This group got its start in the Precambrian seas, invaded the land more than 400 million years ago, and still thrives today." http://evolution.berkeley.edu/evolibrary/article/0_0_1/cambrian_05 As seen on this diagram, crustaceans appeared at about the same time as trilobites, probably in the sea: http://evolution.berkeley.edu/evolibrary/article/0_0_0/success_03 7) "Those crustaceans that have hard exoskeletons reinforced with calcium carbonate, such as crabs and lobsters, tend to preserve well as fossils, but many crustaceans have only thin exoskeletons. Most of the fossils known are from coral reef or shallow sea-floor environments, but many crustaceans live in open seas, on deep sea-floors or in burrows. Crustaceans tend, therefore, to be rarer in the fossil record than trilobites. Some crustaceans are reasonably common in Cretaceous and Caenozoic rocks, but barnacles have a particularly poor fossil record, with very few specimens from before the Mesozoic era." Source: http://en.wikipedia.org/wiki/Crustacean