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 beneath 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 the great sym- 

 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 the body, or of the second uckler, 

 are analogous to the feet of the same Decapoda. 



f Messrs. Audouin and Edwards have observed in the Maia and in the Palinurus 

 a nerve analogous to the one called Lyonet, in his Anatomic de la Chenille du Saule, 

 4< recurrent." The discovery of the other gastric nerves is also due to them. 



