CROWTHER : BIOLOGY OF SPH^ERIUM CORNEUM. 419 



of preventing much of the waste material of the same animal 

 from commg within the range of the in-flowing currents. 



The protrusion of the foot, to which I have already drawn 

 attention, is best seen in those animals which have been sus- 

 pended for some time; for days together they may remain with 

 their siphons extended to twice the length of their shells, and 

 their foot equally as far, no attempt being made by the animals 

 to use it as an organ of locomotion. The microscope reveals 

 that the foot is ciliated, and that it is assisting the mantle and 

 the incurrent siphon in bringing to their possessor food and 

 air. This adaptation of the foot in these pelecypods as an 

 incurrent ciliated respiratory and alimentative organ is surely 

 an interesting example of those modifications which may occur 

 in animals when conditions are favourable to their develop- 

 ment. A dozen or more of these molluscs dropped like 

 pebbles to the bottom of a small beaker, used the foot only 

 to adjust themselves, or to better their position, and then 

 held vertically their extended siphons, the excurrent of course 

 being diverted at their distal extremities. The extension of the 

 foot was not practised in this case, as the molluscs would 

 have only stirred up matter already cast away. We see, then, 

 in the suspended forms the primary use of the organ of loco- 

 . motion subordinated to another purpose. 



The gills are an ever-charming study. The exploration of 

 their ascending and descending lamellae in the living Sphcerium 

 is productive of many surprises — light-coloured gill filaments 

 with their chitinous rods, lacunar spaces with blood corpuscles; 

 ciha of many shapes and performing different functions come 

 in succession under the eye of the observer. The stout cilia, 

 which in some forms of pelecypods interlock and bind gill 

 filaments together, here are separate, though playing between 

 each other at places as the fingers of our hands may do, 

 and at other places are wide apart, encircling oval fenestrae. 

 They run down the edges of the filaments in great numbers, 

 until they are replaced by smaller and quicker-moving cilia on 



