PODOSTOMATA. 297 



peculiar in form and also ensheathed by arteries, and the 

 peculiar nature of the gills of the abdominal feet, as well as 

 the highly developed system of blood-vessels; that we are 

 obliged to place it with the Trilobites in a division by itself. 



Fig. 264. A, Brain and eye of a normal Cambarus from Iowa. 



B, The same of the blind craw-fish from Mammoth Cave. 



C, Cornea. Packard, del. 



Eecent researches also on its development prove that the 

 Podostomata should form a distinct class of Arthropods, 

 equivalent on the one hand to the Crustacea and on the 

 other to the Arachnida, but from the fact that they breathe 

 like most Crustacea by external gills, we prefer to retain 

 them in a position between the Crustacea and Arachnida. 



Order 1. Merostomata. The only living representative of 

 this order is the king-crab, belonging to the genus Limulus, 

 represented in American waters by Limulus Polyphemus 

 Linn., which ranges from Casco Bay, Maine, to Florida 

 and the West Indies. 



The body of the king-crab is very large, sometimes nearly 

 two feet in length ; it consists of a cephalo-thorax composed 

 of six segments and an abdomen with nine segments, the 

 ninth (telson) forming a long spine. The cephalo-thorax is 

 broader than long, in shape somewhat like that of Apus, 

 with a broad flat triangular fold on the under side. Above 

 are two large lunate compound eyes, near the middle of the 

 head, but quite remote from each other, and two small sim- 

 ple eyes situated close together near the front edge of the 

 head. There are no antennae, and the six pairs of append- 



