STRUCTURE OF ARENICOLA. 



201 



WILSON. 



Two inner primitive Neuroblasts. 

 cells. 



Next three cells on 

 either side. 



Posterior cell of each 

 side. 



/Two nephridioblasts, 

 \ one lateral cell, 



Primitive mesoblast. 



BERGH. 



Neuroblasts. 



f Outer or anterior 

 \ myoblasts. 



Inner or posterior 

 myoblast. 



According to Bergh the germ band consists originally of three cells 

 on each side, the neuroblast, the primary inner myoblast, the outer 

 myoblast. The primary inner myoblast later gives origin to the three 

 inner myoblasts, nephridioblasts, and lateral cell of Wilson. 



Further, Bergh states that at an early stage a "nerve plexus" arises 

 on the mid-ventral line, probably from the ectoderm, and that this 

 unites with the neuroblastic rows to produce the nervous system of the 

 adult. 



Type of POLYCH^TA. The Lob Worm (Arenicola 

 piscatoruni). 



Habits. 



On the flat sandy beach uncovered at low tide, the 

 " castings" of the lob worm are very numerous. There 

 the fishermen seek the worms for bait, and have to dig 

 deep, for the burrowers rapidly retreat far into the sand. 

 The burrows of the lob worm are cylindrical tubes, lined by 

 a yellowish green secretion, and the surrounding sand is 

 often discoloured by some change in which the organic 

 juices reduce the iron constituents to lower oxides. The 

 tubes are at first vertical, and afterwards oblique or hori- 

 zontal. 



The lob worm burrows like the earthworm eating the 

 sand for the sake of the organic particles or small organisms 

 which it contains. The sandy castings, which pass from 

 the end of the food canal, and are got rid of at the mouth 

 of the tube, fall into spiral coils. When getting rid of the 

 castings, the worm lies with its tail upwards and its head 

 downwards, or with its body bent like a bow; when the 



