CLASS ARACHNIDA 



309 



smaller and lack the long tail. They are found throughout this country, 

 but, being of small size, they do not often excite notice. 



Still another type which belongs here is the king crab, or horseshoe 

 crab (Fig. 212), which is a marine animal found along the Atlantic coast 

 from Maine to Yucatan. It differs from other arachnids in that it lacks 

 malpighian tubules and possesses book gills. Book gills are similar to 

 book lungs but the leaves lie exposed on the ventral side of the abdomen, 



Cephalofhorax 



Fig. 212. — King crab, Limulus polyphemus (Linnaeus). 



xH. 



A, dorsal view. B, ventral. 



and oxygen is taken from the water. The king crab, together with a 

 few mites, represents all of the arachnids which are marine. It is noc- 

 turnal, wandering along the shore in shallow water and feeding upon any 

 animal which it can overpower. 



Relatives of king crabs and perhaps their ancestors are the trilobites 

 (Fig. 373). They were present in the earliest known fauna, that of the 

 Cambrian age, and reached their maximum of size, number, and variety 

 in the Silurian (Fig. 371). The largest were nearly two feet long. They 

 disappeared at the end of the Carboniferous, and related types resembling 

 modern crustaceans and king crabs appeared. 



