204 MOLLUSCA. 
holes or spiracles, about sixteen in each row, placed at less 
than the diameter of one apart from each other. Through 
these the water, which flows constantly in at the mouth 
when its orifice is open, appears to be conveyed to the va- 
cant space between the sac and mantle, and it then escapes 
at the funnel. The sac seems extremely thin between the 
spiracles ; but their edges are thickened, as if cartilaginous ; 
and they are lined with closely set ciliae, which, by their 
motion, cause the current of water. When these are in 
full activity, the effect upon the eye is that of delicately- 
toothed oval wheels revolving continually, in a direction 
ascending on the right and descending on the left of each 
oval, as viewed from without; but the ciliae themselves are 
very much closer than the apparent teeth, and the illusion 
seems to be caused by a fanning motion given to them in 
regular and quick succession, which will produce the ap- 
pearance of waves, and each wave here answers to a tooth. 
The spaces between the rows of spiracles are of much more 
substance than the intervals of the spiracles; some liga- 
ments are stretched from them across the side cavities to 
the mantle, which seem intended to keep the branchial sac 
expanded. These spaces also support finger-like processes, 
about eight in a row, that project nearly at right angles into 
the central cavity. 
The central cavity I shall venture to call the mouth, 
though the mouth is said by Cuvier to lie at its bottom. 
The large short tube at its opening ends in five or six ob- 
scure indentations ; it can be drawn in and closed at the 
will of the animal, as can the opening of the funnel. At the 
bottom of the tube the entrance of the mouth is guarded by 
simple tentacula, some longer, some shorter, ranged subal- 
ternately: their number was not ascertained. Whatever 
