146 SCIENCE FROM AN EASY CHAIR 



smooth, and only half as big as one's little finger-nail. 

 The nursing of the young in the gill-sacs is carried to 

 a much further point by Cyclas than by the pond- or 

 river-mussel. Before they are ejected by the parent 

 they are quite large like their parent in appearance, 

 and half as big as a hemp-seed. Necessarily there are 

 not many produced in a season there is not room for 

 more than twenty or thirty young in the gill-sacs. 



