26o 



THE INFANCY OF ANIMALS 



Compare the larvae, for instance, of such sedentary 

 animals as the oyster, that extraordinarily interesting 

 shell-fish known as Yoldia — a near relation of the Razor- 

 shell, and one of 

 the most ancient 

 types of moUusca 

 now living, its for- 

 bears being trace- 

 able into the remote 

 geological past — 

 and the infant 

 stages of the two 

 fresh-water mussels 

 Driessensia and 

 Anodonta. These 

 are all different, 

 as their parents are 

 different, but all 

 are for a time free- 

 swimming. Sooner 

 or later, however, 

 they have to settle 

 down in life, grow 

 a pair of shells, and 

 become respectable. 

 But of these the 

 little anodonta has 

 by far the most ad- 

 venturous existence. 

 For a time it shelters within its parent's gills, but on 

 the first favourable opportunity it is suddenly expelled, 

 with a swarm of its brothers and sisters, to the number 

 of half a million or more at once ! This is freedom, 



YOUNG MOLLUSCA. 



A shows the larval state of Yoldia, a near 

 relation of the Razor-shell. 



B and C show the free-swimming, larval stage of 

 the Oyster. 



D and E are Glochidia — the larvse of the fresh- 

 water mussels Anodonta. Note the teeth on the 

 shell, and the long, vibratile " byssus." 



