144 SCIENCE FROM AN EASY CHAIR 



parents is a very frequent thing in aquatic animals. 

 But there are many devices by which the necessity for 

 such lavish scattering of a new brood is avoided. The 

 common oyster is already a step in advance of the 

 American in this matter, since it protects its young in 

 the very earliest stages within the shelter of its shell. 

 A further advance in this direction is found in the 

 fresh-water mussels (not to be confused with the very 

 different sea-mussels, since they are bitter and tough, 

 and quite inedible, though used as bait in sea-fishing) 

 The pond-mussel (Anodon) and the river-mussel (Unid) 

 are of distinct sexes, and the gills of the female become 

 swollen up at the breeding season so as to form two 

 large bags, into which the eggs are laid by her, as many 

 as 500,000 in number. They are fertilised by the sperm 

 filaments discharged by the males, which are carried into 

 the female's shell by currents produced by the vibrating 

 hairs on the gills, as in the common oyster. But the 

 young remain much longer in the mussel's gills than do 

 the young oysters in those of their parent's. Late in 

 the season you find the bag-like gills of the female pond- 

 and river-mussels full of extraordinary little creatures 

 one-thirtieth of an inch long, each provided with a pair 

 of triangular shells. They are discharged into the water, 

 and swim very actively by rapidly opening and shutting 

 the little shells (Fig. 35). The common scallop (Pecten, 

 or Pilgrim's shell) swims every now and then in the 

 same way as do these young mussels, and so do some 

 other bivalves. The young fresh-water mussels produce 

 a long, sticky thread, which trails from the shell (Fig. 3 5 by). 

 Very few have the good chance to get further on in life 

 than this stage, for all depends on their stumbling across 

 fish a stickleback, or a perch, or a pike as they 

 blindly snap their shells and wobble through the water. 

 The lucky triangular mite whose sticky thread happens 



