188 THE FRESH-WATER MUSSEL [CH. VI 



from the other side of the body and thus greatly diminish 

 or even totally close the gap leading into the space within 

 the inner gill-plates. 



Each egg is contained in a transparent egg-shell from 

 which, prior to fertilisation, there projects a chimney-pot- 

 like tube, the micropyle. Fertilisation takes place within 

 the gill-chamber by a spermatozoon swimming down the 

 micropyle tube, which is thereupon closed and withdrawn ; 

 a wrinkled scar, resembling a " Catherine- wheel," marking 

 the position which it occupied. The egg then undergoes 

 segmentation and at length the stage known as the 

 " veliger " is reached. This very characteristic molluscan 

 larva is provided with a belt of cilia, the velum. In the 

 marine relatives of Anodonta the veliger escapes from the 

 egg-shell and swims freely in the surface waters of the 

 sea. In the present instance however the larva remains 

 within the egg-shell, but nevertheless for a few hours its 

 cilia become active, causing the animal to revolve slowly 

 in the limited space at its disposal. It is remarkable that 

 this evidence of a marine origin should still persist in a 

 form that has for so long a period inhabited fresh water. 

 Fossil Anodonta are found in the Palaeozoic Rocks as far 

 back as the Old Red Sandstone, the great antiquity of the 

 genus is further attested by its present world-wide geo- 

 graphical distribution. 



The veliger loses its cilia and by October the important 

 stage known as the Glochidium is reached. The young 

 animal now possesses a bivalve shell, each half of which 

 is roughly triangular. The bases are united by a dorsal 

 elastic ligament, and to each ventral apex is attached a 



