896 Something about Crabs. 
by several species (some their near relatives) which occur in tropical 
countries, the warmer portions of America possibly being their 
metropolis. These land crabs are strange creatures, which, like 
their marine relatives, are provided with gills, but which, unlike 
them, stay away from the water for months at a time, and which 
are drowned by submergence almost as quickly as any terrestrial 
mammal. There is a considerable diversity in their habits. Some 
frequent the lowlands near some stream or shore, while others, 
notably in Jamaica and on the Isthmus of Panama, live in the 
dense damp forests which clothe the highest mountains. They are 
not easily caught, for they can run with an agility which almost 
surpasses belief, while their strong pincers have powers of nipping 
which are not to be despised. Usually but a few are seen at a 
time, for during the day they rest in their burrows or in holes in 
the rocks, venturing forth for food only as evening draws near. 
Once a year, however, they are compelled to visit the shore to lay 
their eggs. They then appear in incredible numbers, year after 
year, on almost the same day. They march along in vast armies, 
turning to neither side for any obstacle, but devouring everything 
green in their line of march to the sea. 
In connection with these terrestrial crabs many interesting 
physiological experiments are yet to be made. Years ago it was 
noted that besides the gills, the gill chamber contained a very thick 
and spongy lining membrane, which was of use in respiration, and 
several years later Professor Semper made similar observations 
upon an East Indian species, in which he recognized not only the 
spongy layer, but also ascertained that it was richly supplied with 
vessels; in short, that this organ, so far as function was 
concerned, is a veritable lung, though of course of a far different 
origin from that of the higher vertebrates. 
