182 
microscope, either 7m situ in young transparent specimens 
or after being rapidly dissected out, the otoliths exhibit a 
peculiar quivering and rotatory movement. This motion 
is probably due to the action of the cilia in the entrance 
to the tube which leads to the exterior. 
The otocyst does not appear to be an organ of hearing 
as the animal takes no notice of even loud sounds made in 
its immediate vicinity. It is more probably an organ 
for enabling the animal to appreciate its position in the 
sand or in the water and thereby to direct its movements. 
The Nuchal Organ (figs. 5, 34 and 47) is a U-shaped 
ciliated groove formed by an invagination of the epidermis 
of the posterior part of the prostomium. ‘The epithelium 
of the organ is composed of columnar cells, some of which 
are distinguished as sense cells by the presence of neuro- 
fibrille. The intervening supporting cells are rather 
stouter, and many of them are ciliated. In old specimens 
pigment granules are deposited in many of the cells. 
Beneath the epithelium there is a layer of nervous 
elements in connection with the posterior brain lobes. 
The organ probably has an olfactory function. 
The Eyes (figs. 47 and 48).—-Two to five eyes, one of 
which (the oldest) is larger than the others, are present on 
each side of the prostomium in post-larval stages (figs. 
596 and 59). Only in very young specimens from the sand 
are they visible externally. They may be found in sections 
of specimens up to 70 mm. long by which time they have 
sunk below the epidermis and have become imbedded in 
the mass of ganglion cells on the dorsal surface of the 
anterior cerebral lobes just in front of their point of union. 
They are difficult to find in older specimens owing to the 
increase of pigment in the prostomium, and they are 
apparently wanting in some old specimens. 
The eyes are very simple in structure. Each is com- 
