14 NERVOUS SYSTEM AND SENSE-ORGANS 
ganglia are quite distinct, and smaller than those of the thoracic 
region, in correspondence with the relatively small size of their 
segments. But the last two pairs have united into a somewhat 
larger nerve-mass, which supplies the last two segments of the 
body, that include the large tail-fin. It has been shown by ex- 
periment that the brain of the Crayfish is the dominating centre 
of the nervous system, while the remaining nerve-masses are 
centres of reflex action for the segments which they supply. 
Turning now to the Crab, in which the head and thorax are 
relatively short and broad, and the abdomen insignificant, the 
brain is comparable to that of a Crayfish, but 
all the ganglia of the short ventral cord have 
fused together into a single mass, placed near 
the under side of the thorax, and perforated by 
the artery which runs down from the heart (fig. 
1016). 
In all the three Crustaceans described there 
is a visceral nervous system, the roots of which 
are indicated in the figures. 
Nervous Systems of Atr-breathing Arthro- 
Fig. r0x6.—Central Ner- Pods (Tvacheata).—Comparison of Peripatus, 
foes Pye Ot dewano Myriapods, Arachnids, and Insects will show 
or eset ew, that the same lines are followed as in Crus- 
seen behind) with momerous  taceans. In the less-specialized forms, where 
the body is elongated and there has been -but 
little fusion between segments, the nervous system is very like 
that of an Annelid. But in cases where the body is compara- 
tively short and much fusion has taken place, the nervous system 
is concentrated to a corresponding degree. 
We have already had occasion to see that Peripatus is the 
most primitive of all living air-breathing Arthropods, and has 
the closest affinity to Annelids. This view is fully borne out by 
examination of the nervous system. The upper side of the nerve- 
ring is swollen into a relatively large brain, and the two halves 
of the ventral cord are widely separate, though united by numerous 
transverse bands of nerve-fibres. The outer part of each cord, 
through its entire extent, contains numerous nerve-cells, and these 
are not aggregated into well-marked ganglia (fig. 1017). 
Myriapods, such as ordinary Centipedes and Millipedes, are 
rather more specialized than Peripatus, and possess a well-marked 
