158 ERRANTIA. PAET in. 



double cord extending along the ventral side of the 

 body, and united at equal intervals by double nerve- 

 centres, as in fig. 131 ; but in the Annelids the two 

 cords diverge below the gullet, surround it, unite again 

 above that tube, and form a principal bilobed nerve- 

 centre or brain. Each segment of the worm is occupied 

 by a small double nerve-centre. In some of these 

 marine worms there are hundreds of segments and as 

 many nerve-centres. There are more than a thousand 

 of these pairs of nerve-centres on the ventral cord of 

 the Nemertes gigas, or Great Band Worm, which is 

 sometimes forty feet long and an inch broad. The head 

 is like a snake, and the bristled feet are jointed to enable 

 it to move over hard surfaces. 



The movements of the bristly feet of the Errantia 

 are reflex, depending on the nerve-centres in their seg- 

 ments ; but they are controlled and connected by the 

 double cord which passes through them. 



Every hair, cirrus, and tentacle on the bodies of the 

 Errant Worms is a living organ of feeling, shrinking at 

 the smallest touch, but enabling them to select their 

 food, to move towards and retreat from objects, and to 

 thread their way through the most intricate labyrinths 

 with unerring certainty, which seems to render them 

 independent of eyes ; yet many of them have multitudes 

 of eyes, or rather eye-specks, according to the genera. 

 Some have but one eye- speck placed in the forehead ; one 

 genus has a double row throughout their whole length, 

 two in each segment, while the Amphicora has two in 

 its tail. All these eye-specks have their crystalline 

 lens, pigment-layer, and nerve-bulb, so that the Errant 

 Worms must see objects, and their motions show that 

 they do ; but we can form no idea of the kind of vision. 



Besides the variety of organs on the skin of the 

 Errantia, some of these worms have two rows of flat 

 plates on their backs overlapping each other at their 



