CRA YFISH. 



287 



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

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

 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 off to the food canal, 

 forming a complex visceral or 

 stomato-gastric system. Simi- 

 larly, from the last ganglia of the 

 ventral chain, nerves go to the 

 hind-gut. If the brain be regard- 

 ed as the fusion of two pairs of 

 ganglia, as the development sug- 

 gests, and the sub-oesophageal as 

 composed of six fused pairs, ther- 

 these, along with the eleven other 

 pairs of the ventral chain, give 

 a total of nineteen nerve-centres, 

 a pair for each pair of append- 

 ages. 



Sensory system. A skin 

 clothed with chitin is not FIG. 143. Section of compound eye 

 likely to be in itself very ^i My sis vulgaris. After Gren- 

 sensitive, but some of the 

 setae are, and some ob- m ' 

 servers describe a peri 

 pheral plexus of nerve-cells 

 beneath the epidermis. 

 The setae are not mere 

 outgrowths of the cuticle, but are continuous with the 

 living epidermis beneath ; and though some are only fringes, 

 both experiment and histological examination 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 specialised setae have sunk into a sac at the base 

 of the antennules, and are spoken of as auditory. The sac 



acher. 



Muscle of eye-stalk ; 



ganglionic 



1-4 iF 



swellings in the course of the optic 

 nerve ; ., the nerve fibrils passing up 

 to the retinulae ; rh. t the rhabdoms ; 

 re., elements of retinulae ; /*., band of 

 pigment ; c., crystalline cones ; co., the 

 corneal facets with the subjacent nuclei. 



