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- (tracheae) 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 the 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. la this central 

 part is situated the alimentary tract, the mouth 

 opening between the bases of the second of the 



