ARTHROPODS. CLASS ARACHNIDA 139 
other peculiarities have arisen which fit them for their 
different modes of life. 
_ 185. The king crab (Limulus).—The king crab may be 
found crawling over the bottom or plowing its way through 
the sand and mud in many of the quiet bays from Maine 
to Florida. The large head and thorax of these animals 
are united into a horse- 
shoe-shaped piece, be- 
hind which lies the 
triangular abdomen. 
On the curved front 
surface of the former 
are a pair of small me- 
dian eyes, and farther 
outward are two larger 
compound ones. On 
the ventral side are 
six pairs of append- 
ages, Instrumental in 
capturing and tearing 
the small animals that 
serve as food, and 
functioning in con- 
nection with the ter- 
minal spine as locomo- Fie. 86.—The king or horseshoe crab (Limulus 
polyphemus). 
tor organs. On the 
ventral surface of the abdomen are numerous plate-like flaps 
which serve in respiration, and in the imperfect swimming 
movements in which these animals occasionally indulge. 
These relatively large and clumsy creatures are the rem- 
nant of a great number of strange, uncouth animals that in- 
habited the earth in past ages. Many of them show a close 
resemblance to the scorpions. The anatomy and develop- 
ment also show certain points of resemblance, and by some 
are thought to give us an idea of the ancient type of spider- 
like animal from which the modern forms have descended. 
