CRA VFISH. 287 
ventral chain is double, and at one place, between the fourth 
and fifth ganglia, an artery (sternal) passes between the two: 
halves of the cord. From each pair of ganglia nerves are 
given off to appendages and muscles, and apart from the 
brain these minor centres are able to control the individual 
movements of the limbs. In the thoracic region the cord is 
well protected by the cuticular archway already referred to. 
From the brain, and from the 
commissure between it and the 
sub-cesophageal ganglia, nerves 
are given off to the food canal, 
forming a complex visceral or 
stomato-gastric system, Simi- 
larly, from the last ganglia of the 
ventral chain, nerves go to the 
hind-gut. If the brain be regard- 
ed as the fusion of two pairs of 
ganglia, as the development sug- 
gests, and the sub-cesophageal as 
composed of six fused pairs, ther 
these, along with the eleven other 
pairs of the ventral chain, give 
a total of nineteen nerve-centres, 
—a pair for each pair of append- 
ages, ‘ 
Sensory system.—A skin ‘ 
clothed with chitin is not Fic. 143.—Section of compound eye 
likely to be in itself very of Adyses vulgards.—After Gren- 
sensitive, but some of the cher. ae 
sete are, and some Ob- “Jdllings in the course ‘of the® optic 
servers describe a perl- nerve; 7.5. the nerve fibrils passing up 
to the retinule; +/4., the rhabdoms; 
pheral plexus of nerve-cells ve., elements of retinule; 4., band of 
beneath the epidermis. ements ca crete ee sacle 
The sete are not mere 
outgrowths of the cuticle, but are continuous with the 
living epidermis beneath ; and though some are only fringes, 
both experiment and histological examination show that 
others are ¢actdle. 
On the under surface of the outer fork of the antennules 
there are special innervated sete, which have a smelling 
function. 
Other specialised setee have sunk into a sac at the base 
of the antennules, and are spoken of as auditory. The sac 
