Amphipoda is an order of crustaceans with no carapace and laterally compressed bodies. Amphipods range in size from 1 to over 300 mm and are, for the most part, scavengers. There are more than 9,000 amphipod species so far described. They are mostly saltwater dwellers but can found in freshwater environments, with some 1900 species found there, the order also includes the beach hoppers such as talitrus saltator.
The body of an amphipod is divided into 3 segments, head, thorax, and abdomen. The head is joined to the thorax and has two pairs of antennae along with one pair of compound eyes. The thorax and abdomen are usually quite distinct and bear different kinds of legs, they are typically laterally compressed, and there is no carapace. The thorax bears eight pairs of appendages, the first of which are used as accessory mouthparts, the next four pairs are directed forwards, and the last three pairs are directed backward. The abdomen is divided into two parts, one with swimming legs and the other with back legs for jumping.
Mature females bear a brood pouch, which holds their eggs, where the male will fertilize them. They stay in this pouch until they are ready to hatch. As females age, she produces more eggs in each brood. The eggs will hatch directly into a juvenile form and sexual maturity is reached in about six months.
Some Amphipods can get to be pretty big, the one on this page was photographed up at San Josef Bay, on Northwest Vancouver Island. It was about 2 cm long, quite big for an amphipod for sure. I have seen some on the west coast that have been even larger. This one was called a beach hopper and they are very common on the west coast shores of Northern Vancouver Island. They can also be seen on the east coast of the island, although smaller in size and not very common, I was walking along a beach with my daughter-in-law up in Sayward and she spotted one.