81 
380 grains: the brain and spinal marrow 3 grains : 
the brain singly 1 grain. The great hemispheres 
of the brain are smallest in the batrachia, larger in 
the chelonia and ophidia: most fully developed in 
lizards, iguana, and crocodile.” 
In fishes the spinal marrow preponderates in bulk 
over the brain. It is generally longer than in the 
former classes, extending through the caudal ver- 
tebree. Yet it is in some peculiarly short, as in te- 
trodon, mola, and lophius piscatorius. 
In the crustacea is found a chain of ganglia, or 
knots, united by a central thread. From the fore- 
most of these proceed branches surrounding the ceso- 
phagus, and nerves for the eyes, mouth, antenne, 
&e. the cerebral ganglion being in some larger, in 
others less. 
Spiders and scorpions form a medium of transi- 
tion from crustacea to insects. Thus the nervous 
system of aranea diadema may be compared with 
that of the crab”; for in it also, besides the cerebral 
ganglion, a large nervous mass is found in the tho- 
rax, giving off nerves for the legs, and from which 
a nervous cord passes backward to a postern gan- 
glion. “ In the parasitic genera, pediculus and rici- 
nus, the cerebral ganglion consists of two oval lobes ; 
and instead of two cords surrounding the cesopha- 
gus, it is elongated into a single cord, on which 
there are three ganglia closely approximated. In 
the diptera, neuroptera, lepidoptera, &c. the nervous 
> Carus, vol. ii. passim. 
G 
