298 ZOOLOGY. 
ages are of uniform shape like legs, not like mandibles or 
maxille, and are adapted for walking; the feet are pro- 
vided with sharp teeth on the basal joint for retaining the 
food. The mouth is situated between the second pair; the 
first pair of legs are smaller than the others. All end in 
two simple claws, except the sixth pair, which are armed 
with several spatulate appendages serving to prop the crea- 
ture as it burrows into the mud. The males differ from the 
females in the hand and opposing thumb of the second pair 
of feet. These cephalo-thoracic appendages are quite as dif- 
ferent from those of most Crustacea as those of the mites 
and spiders, which have a pair of mandibles and maxille, 
the latter provided with a palpus. Appended to the ab- 
domen are six pairs of broad swimming feet, all except the 
first pair of which bear on the under side two sets of about 
one hundred respiratory leaves or plates, into which the blood 
is sent from the heart, passing around the outer edge and 
returning around the inner edge. This mode of respiration 
is like that of the Isopods., 
The alimentary canal consists of an cesophagus, which 
rises directly over the mouth, a stomach lined with rows of 
large chitinous 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, ramifying throughout the cephalo- 
thorax. The nervous system is quite unlike that of the 
Crustacea ; the brain is situated on the floor of the body in 
the same plane as the rest of the system, and sends a pair of 
nerves to the compound eyes, a single nerve supplying the 
ocelli.* The feet are all supplied with nerves from a thick 
ring surrounding the csophagus. The nerves to the six 
pairs of abdominal legs are sent off from the ventral cord. 
* The nervous system of Limulus is quite unlike that of the Scorpion, 
where the brain is situated in the upper part of the head and supplies 
the maxille with nerves, and is situated directly over the infraceso-~ 
phageal ganglion; and, besides, there is no cesophageal ring as in 
Limulus, only the two commissures connecting the brain with the 
infracesophageal ganglion as usual in the Crustacea and Arachnida in 
general. 
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