NERVOUS SYSTEMS OF INVERTEBRATES II 
In the higher members of the group, ¢.g. lobsters and insects, the 
segments in front have fused into a well-developed head, followed 
by a thorax, to constitute which other segments have coalesced, 
while this is succeeded by an abdomen, where the amount of union 
of segments varies greatly in different cases. These three suc- 
cessive regions of the body differ greatly from one another as to 
size and shape, and may undergo further fusion. Thus, in a 
A PROBOSCIS PORE __PROBOSCIS IN SHEATH 
mye = — SS 
LO C2] = . ST SSS 
(Le 7 
i 
ce BRAIN MOUTH cut LATERACNERVE 
HEADSLIT\\ pennanee 
——— Za 
— DORSAL NERVE 
Yr PROBOSCIS SHEATH 
Vy 7 MMM : 
( oe Y/ ROBOSCIS 
‘B. ROBOSCIS PORE 
LATERAL NERVE 
Tl. 
_—-NERVE RING 
BRAIN 
DORSAL NERVE 
LATERAL NERVE 
PROBOSCIS 
IN SHEATH Za LATERALNERVE 
Fig. 1013.—Diagrams to illustrate Structure of a Nemertine Worm, represented as a transparent object 
A., Side view; B., front end, seen from above; C., cross section. 
Lobster, head and thorax are welded together, and in a Spider 
not only is this so, but the abdominal segments have closely united 
into a rounded mass. 
The nervous system of an Arthropod, like that of an Annelid, 
consists of nerve-ring and double ventral nerve-cord, but the 
ganglia are better developed, and in the higher members of the 
group they are more or less fused together into larger nerve- 
masses, just as the segments to which they belong are similarly 
united. There is, in other words, an increasing amount of cen- 
tralization in the nervous system as we pass from lower to higher 
forms in any subdivision of the Arthropods. And this is clearly 
