33° 



PHYSIOLOGY OF THE INVERTEBRATA. 



In some Crustacea, suck 

 as the shore-crab (Carcinus 

 mmnas), there is a large cere- 

 bral ganglion which gives 

 off nerves to the eyes and 

 antennae ; while the ventral 

 chain of ganglia (of other 

 forms) is fused into one 

 mass (Pig. 64). Prom this 

 mass radiate the nerve-cords. 

 The nerve-cords connecting 

 the cerebral ganglion with 

 the nervous mass form the 

 oesophageal ring or collar. 

 There is in Carcinus a degree 

 of concentration of the gan- 

 glionic cells, greater, in some 

 respects, than in the Verte- 

 brates themselves. 



The nervous system of 

 Asta^us fiuviatilis (Fig. 65, 

 and see also Fig. 13) con- 

 sists of thirteen ganglia 

 joined together by means of 

 commissures. These ganglia 

 are divided as follows: one 

 cerebral, one sub-oesopha- 

 geal, five thoracic, and six 

 abdominal ganglia. The 

 cerebral ganglion or brain 

 gives oflF nerves to the eyes ; 

 to the auditory organs; to 

 the antennae ; to the cara- 

 pace in front of the cer- 

 vical suture; to the green 

 glands; to the visceral nervous system; and to the sub- 



FlG. 65. 



Nervous System of Astacus. 

 a = brain. !> = optic nerve, c = " collar." 



d = sub-cesophageal ganglion, 

 e = visceral nerve. / = posterio-lateral 



nerve. g = " hepatic " nerve. 



The stomach is turned on one side to show 



its nerves. 



