99 



cylindrical sac, common to most of the Rotlfera, though I know of 

 no other example of this dense body occupying it. The sac is 

 capable of much motion, quite independent of that of the gizzard, 

 sometimes being retracted, sometimes drawn towards the ventral 

 surface, or to either side. There is a tube running through the 

 centre of this occipital sac, which widens at its lower end to embrace 

 the white mass. The latter is arranged in numerous lobes, some- 

 times very distinct (as in A), at others more consolidated (as in B), 

 and the whole mass occasionally contracts irregularly, when the 

 matter, in the form of dense granules, is partially forced up into the 

 tube, again falling when the mass expands. That this is a great 

 ganglion or cerebral mass of nervous matter, is probable, from 

 its being the seat of the great coloured eye ; although we certainly 

 do not expect to find such a concentration of neurine in an animal 

 of this low grade : at all events, I shall take the liberty of so 

 designating it. At times the eye is scarcely perceptible, then 

 suddenly it becomes visible like a flush of crimson, rich in hue, but 

 rather pale, ill-defined (B), lying on the summit of the ganglion. In 

 other specimens its form is very distinct, like a convex lens of ruby, 

 just beneath the widened base of the tube (A). It does not change its 

 form or position, even when the cerebral granules are forced up the 

 tube, to which it seems to present no obstacle. Under strong pres- 

 sure the pigment escapes, and resolves itself into three or four 

 minute red globules, not quite alike in size. 



The ganglionic tube, after pressing through the centre of the occi- 

 pital sac, appeared in one specimen to make an abrupt bend, turning 

 down towards the interior of the body. The walls were not visible, 

 it is true, but the motion of the dense granules, projected or re- 

 tracted in it, exactly like the mercury in a thermometer if you pre- 

 sent it to the fire or remove it, distinctly showed the form as they 

 moved backward or forward. Yet, as I afterwards found, that the 

 direction of the curve is towards the front of the head (as in D), where 

 it can no longer be traced, this apparent retrogression was probably 

 only an optical illusion, caused by the front being bent much for- 

 ward, while the view was taken from behind. 



There may often be seen (figs. H and I) running round the front of 

 the head, parallel with the skin, a tube (1) which sends off irregular 

 branches inwardly that open into another tube. It is not always 

 visible, and I have not been able to trace the termination on either 

 side. It does not appear to have parietes, but to be hollowed in the 



