THE hermit crabs or Anomura are soft-bodied and so for protection use empty gastropod x shells as houses. The gastropod is spiral and, consequendy, modifications in the body form of the Crustacea have taken place. In habit they are extremely active and may be seen running over the seaweed-covered stones in rock-pools, carrying their acquired shelters with them. When alarmed they withdraw into this shelter. One of their pincer-claws is much larger than the other, and serves as a door to close the entrance to the shell when they are inside. Not infrequently the shell carries a sea-anemone, which shares the crab’s meals and, in return for board and lodging, protects the crab by means of its stinging cells. A little crab, which lives in the Indian Ocean, actually carries a living sea-anemone in each claw and uses them as weapons.
The true crabs are usually broader than they are long, the abdomen being merely a small flap which is folded under the thorax, and there is no terminal tail fan. The abdominal appendages are only developed to any extent in the female, where they act as organs of attachment for the numerous eggs —a condition spoken of as ‘being in berry.’ The life-history of the crab is complicated and consists of three main stages, two of which are larval. The typical larva is called the zoea. It is a characteristic helmet-shaped creature, with large spines and foot-jaws, slender, ringed abdomen and tail-fork. This becomes a mcgalopa larva which resembles the adult except for the abdomen, which is large and bears true swimming legs. The zoea and megalopa are surface-living stages, at which these creatures are often obtained in tow nets. The adult crab develops from the megalopa larva.