254 



CRUSTACEA. 



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- 

 oesophageal ganglia, nerves are given off 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 

 seta? are, and some ob- 

 servers describe a peri- 

 pheral plexus of nerves 

 beneath the epidermis. 

 The setae are not mere out- 

 growths of the cuticle, but 

 are continuous with the 

 living epidermis beneath; 

 and though some are only 

 fringes, both experiment 

 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 



Fig. io6a. — Section of compound eye 

 of Mysis vulgaris. — After Gren- 

 adier. 



m., Muscle of eye-stalk ; 1-4 ganglionic 

 swellings in the course of the optic 

 nerve ; n., the nerve fibrils passing up 

 to the retinulse ; rh., the rhabdoms ; 

 re., elements of retinulse; p., band of 

 pigment; -c., crystalline cones ; Co., the 

 corneal facets with the subjacent nuclei. 



