270 PHYL UM AR THROPODA. 



the greater part of the mandible is obviously well 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 

 (epistoma) in front of the mouth, and the slight upper and lower lips ; 

 and the lateral flaps of the body wall which protect the gills. Each 

 posterior segment consists of a dorsal arch (fergiun), side flaps (pleura), 

 a ventral bar (sternum), while the little piece between the pleuron and 

 the socket of the limb is dignified by the name of epiineron. The 

 hindmost piece (tctsoit), 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, protecting the 

 ventral nerve-cord and venous blood sinus, and, above all, constituting 

 the complex, apparently, but not really, internal, " endophragmal " 

 skeleton of the thorax. 



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 (apodeniata). 

 The most important sets are (i) 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-oesophageal 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, antennae, and food 

 canal, besides the commissures to the sub-cesophageal 

 centres. They act as a true brain. 



The suboesophageal 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 

 ventral chain is double, and at one place, between the fourth 

 and fifth ganglia, an artery (sternal) passes between the two 



