158 
CRUSTACEA. 
the two middle ones furnishing from its anterior margin an optic 
nerve that plunges directly into the pedicle of the eye and there 
divides into numerous filaments, each of which is destined to a facet 
in the cornea of that organ. The inferior face of the brain produces 
four nerves, which belong to the antennae, and that also give off 
some twigs to the neighbouring parts. Two nervous and very long 
cords, embracing the esophagus laterally and uniting beneath it, arise 
from its posterior margin. There, as in the Brachyura, this union 
only takes place in the middle of the thorax, the medulla then as- 
suming the form of a ring whose proportions are eight times larger 
than those of the brain : six nerves on each side arise from this ring ; 
the anterior ones belong to the parts of the mouth, and the five others 
to the five feet of the same side. From the posterior margin arises 
another nerve which runs to the tail, without producing any sensible 
ganglion, and that apparently represents the ordinary nervous cord. 
Here, as in the Macroura, each of the two nervous cords, previous 
to uniting beneatli the esophagus, and at about the middle of its 
length, gives off a thick nerve for the use of the mandibles and their 
muscles. United, they form a first — sub-cervical — ganglion, that 
distributes neves to the maxillae and the foot-jaws;* they afterwards 
continue approximated throughout their length, presenting eleven 
successive ganglions, each of the five first furnishing nerves to as 
many pairs of feet, and the remaining six those of the tail ; that of 
the Pagurus has some ganglions less, thus appearing to form the 
passage from the Brachyura to the Macroura. M. Serres thinks that 
he has recognised in these Decapoda, vestiges of tlie great syin- 
pathetic f . 
The lateral margin of the shell is bent under, to cover and pro- 
tect the branchiae, leaving an opening anteriorly for the passage of 
water. Sometimes, — see Dorippe — the posterior and inferior extre- 
mity of the thorax has two peculiar apertures for that purpose. The 
branchiae are situated at the origin of the last four foot-jaws and 
feet; the four anterior ones have less extent. The six foot-jaws are 
* According to M. Straus, the anterior division of the body of the Limuli, that 
■which is covered by a semi-lunar buckler, presents, besides the brain, no other 
ganglion but this, whence ■we may infer that the inferior organs of locomotion 
correspond to the parts of the mouth in the Decapoda, Stomapoda, and even in the 
Arachnides, and that those of the other division of thebody, or of the second uckler, 
are analogous to the feet of the same Decapoda. 
Messrs. Audouin and Edwards have observed in the Maia and in the Palinurus 
a nerve analogous to the one called Lyonet, in his Anatomie de la Chenille du Saule, 
“ recurrent.” The discovery of the other gastric nerves is also due to them. 
