222 PHYLUM ANNELIDA. 



growing. The mother-cells are apparently carried backwards as the 

 embryo lengthens, leaving a trail of daughter-cells behind them. The 

 cells so formed also divide, the embryo rapidly lengthening and finally 

 becoming vermiform. Of the eight rows the innermost on each side 

 (neuroblasts) give rise to the nervous system, the next two rows on 

 either side (nephridioblasts) form parts of the nephridia (Fig. 114 (4)), 

 while of the fourth row nothing definite is known. Each row, ending 

 behind in a single cell, widens out and deepens as it is traced forwards. 

 The neural and mesoblastic rows can be traced round the mouth, and 

 help to form the prostomium ; the others fade away at the sides of the 

 stomodcieum. 



Let us sum up this complex history : 



!(a) The original outer layer 



becomes the epidermis. 

 7) The secondary inner strat- 

 Epiblast } um consists of neuroblasts 

 or ( which form the nervous 

 ectoderm. 1 system, of nephridioblasts 

 which form parts of the 

 nephridia, and of lateral 



Type of POLYCH^TA. The Lob-worm (Arenicola 

 marina) 



Habits. On the flat sandy beach uncovered at low tide, 

 the " castings " of the lob-worm or lug-worm are very 

 numerous. There the fishermen seek the worms for bait, 

 and have to dig quickly, for the burrowers retreat one to 

 two feet into the sand. The burrows are curved tubes, 

 lined by a yellowish green secretion from the animal's 

 epidermis, and the surrounding sand is often discoloured by 

 some change which the secretion effects on the iron oxides 

 and other constituents. The tubes are at first vertical, 

 afterwards oblique or horizontal, and then turn vertically 

 upwards again. 



The lob-worm burrows like the earthworm, not only 

 forcing the anterior part of its body onwards, but eating the 



