16 NERVOUS SYSTEM AND SENSE-ORGANS 
plified. Some of the simpler forms possess a nervous system very 
much like that of a short Centipede, and from this condition all 
degrees of fusion and concentration are found, the maximum being 
reached where all the ganglia of the ventral cord have united into 
a single nerve-mass, precisely as in Crabs and Spiders. Three 
such stages, as exemplified by a Termite, a Water-Beetle, and a 
Fly, are represented in fig. 1019. In those insects which begin life 
as larvae, it commonly happens that in this early stage of life the 
nervous system is simpler than in the adult, exhibiting less fusion 
and concentration. This is exemplified by comparison of a cater- 
pillar with the butterfly or moth 
“4 which it becomes, or a bee-grub: 
with an adult bee. Cases are 
known, however, where the ner- 
vous system is condensed both 
in larva and adult, eg. the 
House-Fly and its allies (AZus- 
cide). A curious reversal of 
the ordinary state of things is 
found in the Ant-Lion (AZyr- 
meleo), for here the nervous. 
Fig. 1or9.—.Central Nervous Systems of a Termite < 
(Termes, a), a Water-Beetle (Dytiseus, 8), and a Blow. System of the relatively short 
Cinta and, squat Jarve, is mare con- 
centrated than that of the elon- 
gated adult. That this should be so is probably not merely due 
to difference in shape, for the complex habits of the rapacious 
larva involve elaborate adjustments to the surroundings, which 
need an efficient and centralized nervous system for their proper 
performance (see vol. ii, p. 111). So far as we know, the life of 
the adult is relatively simple. 
It remains to be added that all the air-breathing Arthropods. 
possess a visceral nervous system, which may attain considerable 
complexity, and takes origin from the nerve-ring. 
Nervous Systems or Mottuscs (Motiusca).—The least con- 
centrated type of nervous system is found, as might be expected, 
among some of the Primitive Molluscs (Amphineura). The 
central nervous system of a Mail-Shell (Chzton), for instance, 
consists of a nerve-ring from which four thick nerves run back 
(fig. 1020). Two of these are pedal cords, that traverse the 
substance of the muscular foot, while the others are /ateral cords 
y 
