NERVOUS SYSTEM OF THE LOBSTER 



179 



nal 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 ap- 

 pendages, of which those of the giant chelae are the most highly 

 developed. Still others manipulate the gastric mill, theeyes, 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 which, however, are dor- 

 sal 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-cesoph- 



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, 



74 ._ The otocyst of the lobster 



the double nerve COrd splits opened, showing sensory hairs and otoliths. 

 ,1,1 i (From Gerstaecker after Farre.) 



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- 

 sists of more or less isolated sensory cells with their nerve 



