NERVOUS SYSTEM OF THE LOBSTER 



175 



inal segments are straightened out. These flexor and extensor 

 muscles thus act quite differently from the dermal musculature 

 of the earthworm. Other important muscles work the various 

 appendages of which those of the giant chelae are the most 

 highly developed. Still others manipulate the gastric mill, the 



eyes, etc. 



THE NERVOUS SYSTEM. In general arrangement the nervous 

 system of the lobster is strik 

 ingly similar to that of the 

 worm; here again we find a 

 ventral chain of nerve gan- 

 glia lying immediately dorsal 

 to the ventral blood vessel. 

 A pair of cerebral ganglia 

 close to the eyes, innervates 

 these and adjacent organs. 

 A long pair of circumoe- 

 sophageal commissures con- 

 nects the cerebral with the 

 first ventral or sub-oesoph- 

 ageal ganglia. These, how- 

 ever, represent a fusion 

 of thoracic ganglia for just 

 as the somites here have 

 merged to form the cephalo- 

 thorax so these ganglia have 

 fused into one. Between 

 the fourth and fifth ganglia 

 the double nerve cord splits FlG 

 and allows the sternal artery 

 to pass through. In the abdomen the nerve chain is quite 

 regular and similar to that of the earthworm, in having one 

 pair of ganglia to each somite (Fig. 73). 



SENSE ORGANS. In the earthworm we have seen that there 

 is a well-marked advance in nerve-organization over forms like 

 Hydra, with grounds for dividing it into peripheral sensory and 

 internal central nervous systems. The peripheral system con- 



74. The otocyst of the lobster 

 opened. (From Gerstaecker after Farre.) 



