CH. VI] THE FRESH-WATER MUSSEL 187 



the process, and since the latter is prolonged for several 

 days it seems probable that the animal would be suf- 

 focated. 



I have, however, observed 1 on several occasions a violent 

 and sudden reversion of the water-currents such as would 

 certainly be fully capable of carrying the eggs forward 

 and into the latticed recesses of the outer gills. This 

 reversion is caused by the animal, firstly, closing all the 

 ventral border of the shell by means of the free edges of 

 the mantle assisted by the flexible, uncalcified rim of 

 periostracum and leaving the siphons alone open, and 

 secondly, relaxing the adductor muscles so as to allow 

 the elastic ligament to make the valves gape apart. 

 These actions cause the hydrostatic pressure within the 

 shell to be less than that of the water without and 

 consequently there ensues a rush of water into the shell 

 through the open siphons. The whole procedure may be 

 likened to a gulp and is achieved by precisely similar 

 physical forces. 



It may be enquired why the eggs do not also 

 find their way into spaces of the inner gill. This 

 is due partly to the fact that the space between the 

 lamellae of the outer gill-plate is greater than that 

 between those of the inner and partly to the blocking 

 of the latter by the following arrangement. Posterior to 

 the foot the inner lamella of the outer gill-plate extends 

 further towards the dorsal surface than the outer lamella 

 of the inner gill-plate, and stretches over towards the 

 middle line so as to nearly or completely meet its fellow 

 1 Cf. Lloyd, Ann. Mag. Nat. Hist. 1870. 



