2/2 



PHYLUM ARTHROPOD A. 



(Cl 



halves of the cord. 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 oft' to the food canal, forming a 

 complex visceral or stomato-gastric system. Similarly, from the last 



ganglia of the ventral chain, 

 nerves go to the hind-gut. If 

 the brain be regarded as the 

 fusion of two pairs of ganglia, 

 as the development suggests, 

 and the sub-cesophageal as com- 

 posed of six fused pairs, then 

 these, along with the eleven 

 other pairs of the ventral chain, 

 give a total of nineteen nerve- 

 centres, a pair for each pair of 

 appendages. 



Sensory system. A skin 

 clothed with chitin is not 

 likely to be in itself very 

 sensitive, but some of the 

 setae are, and some ob- 

 servers describe a peri- 

 pheral plexus of nerves 



FIG. 125. Section of compound eye beneath the epidermis, 

 of Mysis vielgaris. Miev Gren- The setae are not mere out- 

 acher - growths of the cuticle, but 



;;/., Muscle of eye-stalk; 1-4 ganglionic continuous with the 



swellings in the course of the optic a /^ [UlUUUUb WHil 



nerve; ., the nerve fibrils passing up living epidermis beneath ; 

 to the retinulae : r/i., the rhabdoms ; i Ti i i 



re., elements of retinulae; /., band of and though SOme are Only 



pigment; c., crystalline cones; ., the fringes, both experiment 

 corneal facets with the subjacent nuclei. , , 1-1 



and histological examina- 

 tion show that others are tactile. 



On the under surface of the outer fork of the antennules 

 there are special innervated setae, which have a smelling 

 function. 



Other likewise specialised hairs have sunk into a sac at 

 the base of the antennules, and are spoken of as auditory. 

 The sac opens by a bristle-guarded slit on the inner upper 

 corner of the expanded basal joint, and contains a gelatinous 



