THE TRIL0BITE8. 
under side a set of about one hundred respiratory leaves or plates, 
into wliich ibe blood is sent from tbe heart, passing ai'ound the outer 
edge and returning around tbe inner edge. 
In order to examine tbe internal anatomy the student can readily 
with a knife cut tbe body into transverse sections, as in Figs. 120, 
121, and also divide it longitudinally so as to show the parts as in 
Fig. 123. 
The alimentary canal consists of an oesophagus, which rises 
directly over tbe mouth, a stomach lined with rows of large chiti- 
nous teeth, with a large conical, stopper like valve projectij^g into 
the posterior end of the body; the intestine is straight, ending in the 
base of the abdominal spine. Tlie liver is very voluminous, ramify- 
ing throughout tbe ceplialothorax. The nervous system is quite un- 
like that of other Crustacea; tbe brain is situated on tbe floor of the 
body in the same plane as the rest of the system, and sends off two 
Fig. 122.— Horseshoe Crab. Side view. 
pairs of nerves— a pair to each pair of eyes. Tbe feet are nil sup- 
plied with nerves from a thick nervous ring surrounding the oesoph- 
agus. The nerves to the six pairs of abdominal legs are sent off 
from tbe ventral cord. 
The eggs of the horseshoe crab are ratlier 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 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. TriloUta. — The members of this group are all 
extinct. The body has a thick dense skin like that of 
