ARTHROPODS. CLASS ARACHNIDA 



139 



other peculiarities have arisen which fit them for their 

 different modes of life. 



135. 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- 

 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. 



FIG. 86. The king or horseshoe crab (Limulus 

 polyphemus). 



