Sea spiders have a reduced body size compared to the really long legs they have. The land spider has four pairs of legs while the sea spider has from four to six pairs of long segmented legs. The 600 different species of sea spiders range in size from 1/100 to 20 inches across. Because of their small, slender bodies and legs, the sea spider does not need a respiratory system for gas diffusion. A proboscis allows them to suck the nutrients they need from soft bodied invertebrates. The sea spider's digestive tract extends into their legs and is called the diverticulae. Pycnoginds are so small they have single celled muscles surrounded by connective tissue. The proboscis has fairly limited dorso-ventral and lateral movement. 
All sea spiders reproduce sexually and have separate species, except for one species which is hermaphroditic. The females have a pair of ovaries, and the males have a set of testes, which are located dorsally in relation to their digestive tract. The reproduction involves external fertilization after a brief courtship. When the eggs are laid, the male is the only one who takes care of the eggs and the young. After they hatch the larva has a blind gut, and their body consists of a head and the three pairs of cephalic appendages: the chelifores, palps and ovigers. The sea spider's abdomen, thorax, and its thoracic appendages develop later. So far there have been four types of larvae that have been described. These are the typical protonymphon larva, the encysted larva, the atypical protonymphon larva, and the attaching larva. 
The sea spiders are from the phylum Arthropoda and the class Pycnogonida. The largest sea spider lives in the deep ocean. They are found at times in depths of water up to 7000 meters, but it is more common to find them in shallower waters.
The sea spider grows to be the biggest in Antarctica. They also live all around the world ranging from Australia, New Zealand, and the Pacific coast of the United States to the Mediterranean and the Caribbean Seas. They will also live as far as the north and south poles. They can live in a variety of climates, as well as in either deep or shallow water, and are able to live in both marine and estuarine habitats. Sea spiders do not spin webs, but there is a water spider that does. Sea spiders are not like ordinary spiders you would think of when you think of a spider. The land spider is a fast moving predator, but the sea spider will move so slowly sometimes you cannot even tell it is alive.