SPIDERS, MITES, AND SCORPIONS. 155 



Like all Arthropods the horseshoe molts its skin; but this process 

 differs somewhat from that in the lobster. The shell does not split down 

 the middle, but around the sharp, front edge, thus making a large opening 

 through which the body can be readily extracted. Along the sandy 

 beaches where these forms abound, their cast shells can be found in great 

 numbers, and of all sizes, some large and coarse, others small, delicate, and 

 possessing a certain amount of beauty. The horseshoe possesses great 

 vitality ; conditions which are quickly fatal to most inhabitants of the sea 

 affect them much more slowly. A lobster or a crab is quickly killed by 

 immersion in fresh water ; but Limulus will bear submersion in that 

 strange element for several days. The eggs and the young also share this 

 capacity for life under apparently adverse circumstances. The writer kept 

 about a hundred embryos for three months in less than a pint of salt water, 

 which was not changed once during that length of time. They were kept 

 in a shallow dish, and as the water evaporated, it was replaced by water 

 from the city hydrant. Under these circumstances the water at times 

 must have contained two or three times the percentage of salts that it did 

 at others, and yet this seemed not to affect the crabs in the least. Even 

 more remarkable were the experiences of Dr. Samuel Lockwood, the first 

 to study the development of this interesting form. The jars in which he 

 kept his eggs and embryos passed through the winter without any special 

 care, and several times the temperature went down to freezing ; and yet 

 the next spring some of the eggs which failed to hatch the summer before, 

 discharged their young, just a year, lacking two weeks, from the time 

 when they were laid ! 



Of all the forms living on the surface of the earth to-day, the horseshoe- 

 crab and the brachiopods are among the oldest. Through numbers of 

 geologic ages they have retained their forms unaltered, and some of the 

 existing forms are close counterparts of those which lived in carboniferous 

 or silurian times. These forms, to-day, present us with extraordinary 

 instances of individual capacity to withstand adverse circumstances, and 

 seem to warrant the supposition that the vitality of the race is correlated 

 with that of the individual — that the race has survived because the indi- 

 viduals are able to persist through changes of condition, which would either 

 prove fatal to or cause extensive changes in less favored forms. 



The king-crab is aquatic, and breathes by means of gills; its nearesl 

 living relatives, the scorpions, are terrestrial and are adapted for aerial 

 respiration. On the under side of the long and slender abdomen are 

 the openings to the lung-like pouches, and it is interesting to observe 

 that in position and structure these air-breathing lungs are closely similar 

 to the gills of the king-crab. The body of the scorpion is long and 



