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. me Brain and eye of a normal Cambarus from Iowa. 
The same of the blind craw-fish from Mammoth Cave. 
@ Cornea.—Packard, del. 
Recent 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 antenne, and the six pairs of append- 
