CRUSTACEANS. 617 
condition of pairs. The number of the ganglia is, however, very 
different, and varies from more than sixty im apus to two in the 
short-tailed ten-footed crustaceans. In cancer manas a cephalic 
ganglion is situated above or in front of the cesophagus ; from it arise 
the nerves of the eyes and of the antenne and neighbouring parts. 
From the posterior part of this ganglion proceed the two nervous 
filaments, which form the cesophageal 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 cesophagus; 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’. 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 Maja, 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*. 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 Lec. d’Anat. comp. 1. pp. 316, 317: 
2 See AuDOUIN and MitNE Epwarps Recherches anat. sur le Syst. neFveux des 
Crustacés, Ann. des Sc. nat. XIV. 1828, pp. 76—102, Pl. 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 cesophagus 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 cesophagus 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. 
