CKUSTACEANS. 617 



mdition of pairs. The number of the ganglia is, however, very 

 Ferent, and varies from more than sixty in opus to two in the 

 short-tailed ten-footed crustaceans. In cancer mcenas a cephalic 

 ganglion is situated above or in front of the oesophagus ; from it arise 

 the nerves of the eyes and of the antennas and neighbouring parts. 

 From the posterior part of this ganglion proceed the two nervous 

 filaments, which form the oesophageal ring, send in the middle 

 a nerve to the upper-jaws, and afterwards unite in the middle of 

 the thorax in a ganglion that is excavated in the centre. This 

 annular ganglion is almost eight times larger than the cephalic 

 ganglion in front of the oesophagus ; from its circumference nerves 

 arise that run to the feet and other parts, and, in addition, a single 

 nerve from the back part, that runs in the middle throughout the 

 tail and takes the place of the continuation of the ventral cord, but 

 has no ganglia 1 . In this crab, consequently, the ganglia of the 

 thorax and of the anterior part of the abdomen are represented by 

 a single ganglion, which, however, by the excavation in the middle 

 affords an indication of the lateral divisions. In the genus Maya, on 

 the other hand, the union has attained the highest degree, and the 

 large thoracic ganglion has lost the excavation in the middle. 

 Here then there are only two ganglia present, one for the head and 

 one for the thorax and rest of the body 2 . In the long-tailed ten- 

 footed crustaceans, on the contrary, there are different distinct 

 nervous ganglia in the ventral cord, twelve, for example, in the 



1 CUVIER Lef. d'Anat. comp. n. pp. 316, 317. 



2 See AUDOUIN and MILNE EDWARDS Recherches anat. sur le Syst. neweux des 

 Crustaces, Ann. des Sc. nat. xiv. 1828, pp. 76 102, PI. 2 6. 



A fuller description of the nervous system in different families of the Crustacea 

 would extend beyond the limits of this Handbook. I will here only add, that in 

 Limulus the cephalic and thoracic ganglia coalesce to form a thick elongated nervous 

 ring, which behind the oesophagus is connected by three transverse commissures, and 

 from which backwards a nervous cord arises composed of two closely connected nervous 

 bundles, which in the ventral shield presents inconspicuous gangliform thickenings, and 

 afterwards divides into two lateral portions ; each of these strings then terminates in 

 an elongated semilunar ganglion, at the inferior extremity of which two nerves arise 

 that penetrate the caudiform appendage. On the part of the cephalothoracic ring lying 

 in front of the oesophagus there are two conical swellings forwards, from which the 

 two very long optic nerves arise for the two compound eyes situated above on the 

 dorsal shield. These nerves go first forwards and upwards, then bend outwards, and 

 afterwards go backwards to end at the inside of the eyes. Their length is more than, 

 one quarter of that of the whole animal, the tail included. 



