4 INTERNAL ANATOMY OF INSECTS. 
Radiata, or star-fish and sea-urchins a . The third, the 
ganglionic, is where the nervous system consists of a 
series of ganglions connected by nervous threads or a me- 
dullary chord, placed, except the first ganglion, below the 
intestines, from which proceed nerves to the various parts 
of the body. This system may be considered as divisi- 
ble into two — the proper ganglionic, in which it is gan- 
glionic with the ganglions arranged in a series with a 
double spinal chord. This prevails in the classes Insecia, 
Crustacea, Arachnida, &c, and the improper ganglionic, 
in which it is ganglionic with the ganglions dispersed 
irregularly, but connected by nervous threads, as in the 
Mollusca b . In the fourth, the cerebro-spinal, the ner- 
vous tree may be said to be double, or to consist of two 
systems — the first taking its origin in a brain formed of 
two hemispheres contained in the cavity of the head, 
from which posteriorly proceeds a spinal marrow, in- 
cluded in a dorsal vertebral column. These send forth 
numerous nerves to the organs of the senses and the 
muscles of the limbs. The second consists of two prin- 
cipal ventral chords, which by their'ganglions, but with- 
out any direct communication, anastomose with the spinal 
nerves and some of those of the brain, and run one on 
each side from the base of the skull to the extremity of 
the sacrum. This system consists of an assemblage of 
nervous filaments bearing numerous ganglions, from 
which nervous threads are distributed to the organs of 
nutrition and reproduction c . Its chords are called the 
a Cuv. Anat. Comp. ii. 360. MacLeay Hor. Ent. 201. 
h N. Diet. d'Hist. Nat. xvi. 30G. MacLeay Hor. Ent. 200—. 
c Ibid. 307. The great sympathetic nerves in fishes arc said to 
have no ganglions. Cuv. ubi. supr, 297- 
