766 



CRUSTACEA. 



ganglia, strictly speaking, are united 

 into a single mass of a greatly elon- 

 gated form, and presenting a little 

 way back, like the fourth ganglion of 

 the Palemon, a cleft for the trans- 

 mission of the sternal artery. 



The transition 



Fig. 394. from the Deca- 

 poda Macroura 

 to the Brachyura 

 takes place by 

 the Homola, and 

 certain Anomou- 

 ra,* in which the 

 constantly in- 

 creasing concen- 

 tration of the 

 thoracic nervous 

 centres coincides 

 with the almost 

 rudimentary state 

 of the abdominal 

 ganglionic sys- 

 tem, which is 

 here reduced to 

 a kind of median 

 trunk without en- 

 largements. 



This, too, is 



Cephalo-thoracic por- the disposition 

 tion of the nervous presented by the 

 system of the Pali- nervous system 

 nurus Vulgaris. in the Carcinus 

 mcenas among 



the Brachyura, with this difference 

 only, that the medullary nuclei are 

 rather closer to one another, and 

 more intimately connected.f The tho- 

 racic ganglion has the form of a ring, the cir- 

 cumference of which gives origin to the nerves of 

 the thoracic appendages. The single abdomi- 

 nal cord is in its rudimentary state, in obvious 

 relation with the abdomen itself, and therefore 

 reduced to very insignificant dimensions. 



It is in the Maja,| in fine (Jig. 395), that the 

 nervous system is found in its highest degree of 

 centralization ; for the elements of which the 

 two masses there encountered are composed, are 

 so intimately conjoined, that no trace can be 

 found of their ever having existed indepen- 

 dently, although among neighbouring genera 

 several of them may still be discovered isolated- 

 ly. The cephalic ganglion (a) is a sufficiently 

 faithful counterpart of that of the Lobster. 

 The nervous cords (g) which connect this first 

 portion of the system with the thoracic portion 

 also present the same arrangement as in the Lob- 

 ster ; there are similar mandibular nerves, a like 

 gastric pair, the same transverse band (g ) behind 

 the oesophagus, &c. But the thoracic ganglion 

 (/), instead of the ring which it presents in the 



* Vide Rech. sur Torganiz. et la classific. des 

 Crustaces Decapodes par M. Milne Edwards An- 

 nales des Sciences Naturelles, t. xv. 



t Cuvier, Legons d'Anatomie Comparee, t. ii. p. 



$ Audouin et Edwards, loc. cit. 



Fig. 395. 



Nervous system of the Maja Squinado. 

 a, cephalic ganglion ; b, optic nerves ; c, oculo- 

 motor nerves ; d, nerves of the antennulae ; e, 

 fourth pair of nerves belonging to the integuments j 

 ft nerves of the exterior antennae g, medullary 

 cords uniting the cephalic and thoracic ganglions } 

 g', transverse cord ; h, mandibular ganglion ; h', 

 small nerve belonging to the muscles of the 

 mandible ; i, stomato-gastric nerve ; k, lateral 

 branches of the stomato-gastric nerves j I, tho- 

 racic ganglion ; m, nerves of the maxillae ; n, 

 nerves of the first pair of legs ; o, abdominal 

 nerve ; p, cells of the llancs ; q, arch of the 

 flancs. 



Carcinus mcenas, here appears as a solid circu- 

 lar and flattened nucleus giving origin to the 

 whole of the nerves of the thorax and abdo- 

 men, which radiate from it to the number of 

 nine pairs, and one azygous nerve situated in the 

 median plane. There is nothing very remarkable 

 in the distribution of these nerves, unless it be 

 thatseveral pairs,and amongthe number the first 

 and second, are distributed simultaneously to 

 several rings, which proclaims that in the 

 species which engages us the work of con- 

 centration has extended from the ganglions to 

 the nervous cords. 



Any farther detail in addition to what has 

 now been said would contribute little to our 

 essential knowledge of the nervous system. 

 We have traced it from its commencement in 



