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CLASS CRUSTACEA. 
cornea of these organs. The inferior surface of the brain 
produces four other nerves which go to the antennae, and give 
out some threads to the neighbouring parts. From its pos- 
terior edge spring two nervous cords, very much elongated, 
embracing the oesophagus laterally, and uniting underneath. 
In some, as in the brachyuri, this union takes place only in 
the middle of the thorax, and the medullary substance after- 
wards assumes the form of a ring, and in proportions eight 
times larger than the brain. This ring gives rise, on each 
side, to six nerves, the anterior of which repairs to the parts 
of the mouth, and the five others to the five feet on the same 
side. From the posterior edge proceeds another nerve, re- 
pairing to the tail, without producing sensible ganglia, and 
appearing to represent the ordinary nervous cord. In others, 
as in the macrouri, the two nervous cords, before they unite 
under the oesophagus, give birth each at the middle of their 
length, to a thick nerve, repairing to the mandibles, and their 
muscles. United, they form a first median ganglion, (the 
sub-cervical) furnishing nerves to the jaws and the jaw-feet. 
Approximated afterwards, in their entire length, they present, 
successively, eleven other ganglia, the five first of which give, 
each of them, nerves to as many pairs of feet, and the six 
others furnish those of the tail. That of the paguri has some 
ganglia less, and these Crustacea thus appear to form the pas- 
sage, from the brachyuri to the macrouri. We shall add, that 
M. Serres believes that he has recognized in these decapod 
Crustacea, some vestiges of the great sympathetic nerve. 
The lateral edges of the carapace or test, refold underneath, 
to cover and protect the gills, but leave a vacancy anteriorly, 
for the passage of the water. Sometimes even (see Dorippus ) 
the posterior and inferior extremity of the thorax presents 
lor this purpose, two peculiar apertures. These gills are 
situated at the origin of the last four jaw-feet and feet. The 
anterior four are less extended. The six jaw-feet are all of a 
