350 



ZOOLOGY. 



NOTOPODIUM 



ACICULUM 



NEUROPODIUM 



Fig. 180. TYPICAL PARAPODIUM 

 NEREIS MAGNIFIED. 



the body- wall, flattened in an antero-posterior direction, 

 and symmetrical not only on the two sides of the somite, 

 but each one in itself. A parapodium consists of two 

 similar halves, a dorsal and a ventral, the former called 



the notopodium, the latter 

 the neuropodium because 

 it is nearer to the neural 

 surface and the ventral 

 nerve cord (from notos, 

 back; neuron, nerve; and 

 podium, foot-organ) . Each 

 of these halves consists of a 

 cirrus and two lobes, which 

 are somewhat pointed. 

 Thus the notopodium con- 

 sists of dorsal cirrus, outer 

 and inner lobe ; the neuro- 

 podium of ventral cirrus, 

 outer and inner lobe. Each 



of the inner lobes bears a fan of long chaetae, which are 

 compound or jointed, each consisting of a shaft and a blade. 

 The chaetae are really contained in a chaetigerous sac, which 

 is an invagination of the skin, and extends deeply into the 

 interior of the body. The inner lobe of each half of the 

 parapodium is really double, forming the lips of this chae- 

 tigerous sac. In addition to the ordinary slender chaetae 

 there is in each chaetigerous sac a much thicker and more 

 rigid needle-like chaeta called the aciculum, which projects 

 very slightly externally, but extends further than the other 

 chaetae into the interior of the body and serves for the 

 attachment of the muscles that move the whole bundle of 

 chaetae. The two acicula thus act as an internal skeleton 

 of the parapodium, though of course really belonging to the 

 external cuticle. The cirri are well supplied with nerves 

 and serve as tactile organs. The whole parapodium is an 

 organ of locomotion, acting as a lever in the wriggling 

 movement of the worm over the rough ground of the sea 

 bottom, and in crevices between rocks and stones ; at the 

 same time the lobes are well supplied with blood-vessels 

 and serve as organs of respiration. 



