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 
stomodzeum. 
Let us sum up this complex history :— 
la (a) The original outer layer 
becomes the epidermis. 
6) The secondary inner strat- 
Epiblast um consists of neuroblasts 
or which form the nervous 
ectoderm. system, of nephridioblasts 
which form parts of the 
nephridia, and of lateral 
cells of unknown function. 
Two-layered 
Fertilised Plastecehesr _gastrula | Mesoblast 
ovum. blastula with primitive oe 
‘ mesoblasts. inesodeem Muscle. 
formed from Blood vessels. a 
the division of | Ler parts of nephridia. 
the primitive Reproductive organs. 
“*mesoblasts.” 
Mypeblest Lining of 
endoderm. mid-gut. 
Type of Potycua#ta. The Lob-worm (Avenicola 
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 
