236 ARTHROPODS. 



twentieth segment. The most difficult fact to understand 

 clearly, is that the cuticle of certain mouth-parts (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 

 internal, " endophragmal " skeleton of the thorax. 



Muscular System. — The muscles are white bundles of 

 fibres. On minute examination these show clearly that 

 transverse striping which is always well-marked in rapidly 

 contracting elements. They are inserted on the inner 

 surface of the cuticle, or on internal foldings (apodematd) 

 of the same. The most important sets are, (i) the dorsal 

 extensors or straighteners of the tail ; (2) the twisted ventral 

 flexors or benders of the tail, which have harder work, and 

 are much larger than their opponents; (3) those moving the 

 appendages, of which those attached to the mandibles are 

 conspicuously seen in dissection ; (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-oesophageal ganglia are three-lobed, and 

 give off nerves to eyes, antennules, antennae, and food-canal, 

 besides the commissures to the sub-cesophageal centres. 



The sub-oesophageal 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 3rd and 4th 

 ganglia, an artery (sternal) passes through between the two 

 commissures. From each pair of ganglia nerves are given 

 off to appendages and muscles, and apart from the brain, 

 these minor centres are able to control the individual 

 movements of the limbs. In the thoracic region the cord is 

 well protected by the cuticular archway already referred to. 



From the brain, and from the commissure between it and 

 the sub-cesophageal ganglia, nerves are given off to the 



