100 OUR CARCINOLOGICAL FRIENDS. 
in the nature of the walking legs, in the disposition 
and origin of the nerves supplying the legs, in the 
structure of the eye, and in the manner of develop- 
ment of the embryo. As against this evidence we 
have the presence of true gills, and the absence of 
the peculiar breathing-tubes (trachee) of the scor- 
pion; but whether these important points of dis- 
similarity are of greater value than those of affinity 
above indicated, must still be considered an open 
question. 
The crust of the horseshoe crab is divided into 
three distinct parts, an anterior rounded portion, 
known as the cephalothorax, which protects the 
more vital elements of the animal’s body—mouth, 
stomach, heart, and legs; a median portion, the ab- 
domen, on the under side of which are the gills, a 
series of thin plates disposed in the fashion of the 
leaves of a book; and a greatly elongated spine or 
tail. The cephalothorax carries on its back two 
pairs of eyes, of which the two big eyes situated 
on either side of the shield are compound, while 
the others, small and placed medially to the front, 
are simple. 
Although it would seem from tne general vaulted 
appearance of the shield that the body of the animal 
is of considerable thickness, the reverse is actually 
the case. The under surface of the carapace follows 
closely upon the inner face of the upper surface, 
except along the medial region, where the body- 
space is considerably expanded. In this central 
part is situated the alimentary tract, the mouth 
opening between the bases of the second of the 
