286 PHYLUM ARTHROPODA. 
{t is likely that some of the crowded mouth-parts, e.g. the first 
maxillz, are almost functionless. The hard toothed knob which forms 
the greater part of the mandible is obviously weil adapted to its crush- 
ing work. 
In connection with the skeleton, the student should also notice the 
beak (rostrum) projecting between the eyes; the triangular area 
(éfzs¢oma) in front of the mouth, and the slight upper and lower lips ; 
and the lateral flaps of the body wall which project the gills. Each 
posterior segment consists of a dorsal arch (¢ergzmz), side flaps (pleura), 
a ventral bar (sternum), while the little piece between the A/euron and 
the socket of the limb is dignified by the name of efzmeron. The 
hindmost piece (e/son), on which the food canal ends ventrally, is 
regarded by some as a distinct segment. The most difficult fact to 
understand clearly, is that the cuticle of certain appendages (e.g. 
the mandibles), and of the ventral region of the thorax, is folded inwards, 
forming chitinous ‘‘ tendons” or insertions for muscles, and, above all, 
constituting the complex, apparently, but not really, internal, 
‘“‘endophragmal” skeleton of the thorax, protecting the ventral nerve- 
cord and venous blood sinus. 
Muscular system.—The muscles are white bundles of 
fibres, which on minute examination show clearly that trans- 
verse striping which is always well marked in rapidly con- 
tracting elements. The muscles are inserted on the inner 
surface of the cuticle, or on its internal foldings (apodemata). 
The most important sets are—(1) the dorsal extensors or 
straighteners of the tail; (2) the twisted ventral muscles, 
most of which are flexors or benders of the tail, which have 
harder work, and are much larger than their opponents ; 
(3) those moving the appendages; (4) the bands which 
work the gastric mill. 
Nervous system.—The supra-cesophageal nerve-centres 
or ganglia, forming the brain, have been shunted far forward 
by the growth of the pre-oral region. We thus understand 
how the nerve-ring round the gullet, connecting the brain 
with the ventral chain of twelve paired ganglia, is so wide. 
The dorsal or supra-cesophageal ganglia are three-lobed, 
and give off nerves to eyes, antennules, antennz, and food 
canal, besides the commissures to the sub-cesophageal 
centres. They act as a true brain. 
The sub-cesophageal ganglia, the first and largest of the 
ventral dozen, innervate the six pairs of appendages about 
the mouth. There are other five ganglia in the thorax, and 
six more in the abdomen. 
Though the ganglia of each pair are in contact, the 
