THE TRILO BITES. 97 



under side a set of about one hundred respiratory leaves or plates, 

 into wliicli the blood is sent from the heart, passing around the outer 

 edge and returning around the inner edge. 



In order to examine the internal anatomy the student can readily 

 with a knife cut the body into transverse sections, as in Figs. 12U, 

 121, and also divide it longitudinally so as to show the parts as in 

 Fig. 133. 



The alimentary canal consists of an oesophagus, which rises 

 directly over the mouth, a stomach lined with rows of large cliiti- 

 nous teeth, with a large conical, stopper-like valve projecting into 

 the posterior end of the body; the intestine is straight, ending in the 

 base of the abdominal spine. The liver is very voluminous, ramify- 

 ing throughout llie cepljalolhorax. The nervous system is quite un- 

 like tiiat of other Crustacea; the brain is situated on the floor of the 

 body in tlie same plane as the rest of the system, and sends off two 



Fig. 123. — Horseshoe Crab. Side view. 



pairs of nerves — a pair to each pair of ej^es. Tlie feet are all sup- 

 plied with nerves from atliick nervous ring surrounding the oesoph- 

 agus. The nerves to the six pairs of abdominal legs are sent off 

 from tlie ventral cord. 



The eggs of tlie horseshoe crab are rather large, and laid 

 in the sand between high and low water. Just before it 

 hatches it strikingly resembles a trilobite. After leaving 

 the egg (Fig. 12-i) it swims about on its back or burrows in 

 the sand; at first it has no tail-spine, this growing out at 

 successive moults. In casting its shell the latter splits open 

 in front, so that during the process it appears as if spewing 

 itself out. Specimens a foot long are probably several 

 years old. 



Order 2. Trilobita. — The members of this group are all 

 extinct. The body has a thick dense skin like that of 



