TIIE TRILOBITES 



x 



under side a sel of about one hundred respiratory leaves or pi 

 Into \\ liich the bloo I is sent from the heart, passing around 1 

 edge and returning around the inner ed 



In order to examine the internal anatomy the Btudent can readily 

 with a knife cut the body into transverse seotions, as i 120, 



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

 Fig. 128. 



I'll'- alimentary canal consists <>f an oesophagus, which rises 

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

 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 the cephalolhorax. The nervous system is quite un- 

 like that of Dlher Crustacea; the brain i- situated on the floor of the 

 body iu the same plane as the rest of thi in, and sends oil two 



Fio. 12-.'.— Horseshoe Crab. Side view. 



pain Of nerves— a pair to each pair of eves. The feet are all sup- 

 plied with nerves from a thick nervous ring surrounding the oesoph- 

 agus. The nervestothe Bix pairs ,,f abdominal 1 I oil 



from the ventral cord. 



The eggs of the 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. 124) it swims aboui on its back or burrows in 

 tli" Band; at first it has no tail-spine, this growing out at 

 .-in cessive 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 %. Trilobita. — The members of this group are all 

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



