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 runnel. 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 



