A ^'Living Fossil'^ 227 



low-water mark. The young, however, are often found 

 in considerable numbers on the less exposed floor of the 

 tide pool, where, molelike, they plow their tortuous 

 way through the upper part in pursuit of worms. The 

 adult animal when full grown is nearly two and a half - 

 feet long, and its rounded domelike carapace, or ceph- 

 alothorax, is about a foot wide. Half of the length 

 is taken up by a slender spinelike tail which is jointed 

 to the double-tipped abdomen. The abdomen is one 

 rigid piece, and is edged with a row of small spines. 

 Viewed from the top, the crescentic outline of the 

 creature compares favorably with that of the object 

 after which it receives its popular name. The head 

 region is strikingly similar to that of the trilobite, even 

 to the large compound eyes that are near each side at ^ 

 the top. It has another pair of eyes, but these are 

 very simple in structure and there is no evidence that 

 they are of any use to the creature. They are located 

 at the base of the first spine on the middle ridge. 

 Indeed, it is very probable that the vision of the horse- 

 shoe crab at best is quite crude. Nor is. perfect seeing 

 in its case essential. In its underground life of dark- 

 ness — which is the greater part of its existence — there 

 is little necessity for the use of eyes. 



Notwithstanding that Limulus is one of the oldest 

 inhabitants of the sea, it is a very clumsy swimmer. 

 When it does essay to swim, its efforts are usually re- 

 stricted to gliding in an uncertain manner over the 

 bottom, by a method of locomotion that is best de- 

 scribed as half running and half paddling. This ob- 

 servation applies to the adult; when young it manages 

 this thing far better. Whatever capacity the horseshoe 



