Ballast For Spider Crabs

In eme of the tanks at the New York aquarium there ai-e eight spider oraba The spider ctrab is the decorating crab grown up. _ The deeoratiug crab plants npon its back little sprays and streamers of various marine plaats, which it snips off with its pinoher olaws. It carries these, first to its month, where it seenis to keep a snpply of waterproof glue, and thence to its back. It is supposed to do this for protection from its eneniies, for, half bnried in the mud and with these things risiug froru its back, it is practically invisible. It is possible also that the crab uses the sprigs for adornment, as it often selects growths of bright colors. In its larger growth, after it has come to be called a spider crab, it ceases to decórate itself , though sometimes the seeds of marine plants settleon its back andgrow there, but it never loses the handy use of its pincher claws. The largest of the spider crabs at the aquarium is about 3 inches in length. From side to side its legs spread about eight inches. The bottom of the spider crabs' tank is covered with gravel. The tank was cleaned the other day and all but about three inches of the water drawn off. When the reñlling of the tank was begun, all the crabs gathered around the intake. Crabs and fishes in captivity usually dè this, welcoming the fresh incoining water as a man in' captivity might welcome an incoming current of fresh air. The flow of water through the intake pipes can be regulated at will. Here it was permitted to run in at its natural forcé, which was about that of water flowing from a hose. The flow was brokeu somewhat by the wire strainer over the opening of the intake, placed there to keep little fishes and crustaceans from getting into the pipes, but it was too strong for the spider crabs to face. They began to settle themselves down into the gravel and to pile gravel upon their backs, apparently to weight themselves down so that they could withstand it. On a sandy or muddy bottom they could have buried or partly buried themselves. Here the work was more difficult, but they were equal to it. They would work their legs down into the gravel and then lift them, displacing stones in that rnanner, and with their pincher claws they would piek up stones. Sometimes a crab would piek up two gravel stones at once, one in each claw, and place them both on its back. It might tackle a stone so big as to require both claws in the handling of it and raise that and put it on its back. Some of the stones would roll off down by the crabs' legs, but they were not lost there, for they helped to build up around, and the crab would go ahead and put up others on its back. When the tank was full and the inflow cut off, the crabs went walking about again as nsnaL -