ANODONTA 67 



muscle atrophies and is replaced by the permanent anterior 

 and posterior adductors, and the valves of the permanent shell 

 are formed inside those of the larval shell. At the end of from 

 three to twelve weeks the young Anodonta, now developed 

 beyond the glochidium stage, quits the fish on which it has 

 been parasitic, and continues to grow and develop for another 

 three or four years before it attains to sexual maturity. 



The temporary parasitism of the glochidium larva on fishes 

 is an interesting example of adaptation to the conditions of 

 existence. The slow moving Anodonta, ploughing its way 

 through the mud and sand at the bottom of rivers, is incapable 

 of extensive migrations, and its progeny would soon accumulate 

 in such vast numbers in a given locality that they would be 

 unable to subsist there if they were not provided with some 

 means of dispersal. The glochidium, by attaching itself to the 

 fins of fishes, is carried, it may be, to a considerable distance 

 from its birthplace, and when it quits its host it has a better 

 chance of colonising some fresh ground where the competition 

 for the available food supply is less severe. Fresh- water 

 mussels have been observed to spread rapidly into newly made 

 canals, and their journeys in these new waters can only be 

 accounted for by the opportunities for dispersal afforded by 

 the temporarily parasitic habits of their larvae. 



Marine Lamellibranchs have free swimming ciliated larvae 

 known as veligers. A fresh-water mussel, Dreissensia poly- 

 morpha, allied to Anodonta, has a veliger larva, and its young 

 are therefore more readily dispersed. But the minute ciliated 

 larvae could not swim against the stream, yet Dreissensia has 

 been known to spread from the brackish waters at the mouth 

 of the Rhine up as far as Mannheim, and from Mannheim up 

 the Main into the Main and Danube Canal, and now exists in 

 the Danube itself where it was formerly unknown. In this 

 case, the adult fixes itself by a byssus to various objects, 

 among others to the bottoms of boats, and thus has been 

 carried long distances against the stream. It has been carried 

 in the same manner up the Thames, and is abundant in the 

 Oxford and Birmingham Canal. The two cases are instructive. 

 Anodonta has long been an inhabitant of fresh water, and its 

 development has been profoundly modified in connection with 

 its habitat. Dreissensia has only recently taken to a fresh- 

 water life, and still lives in brackish waters in estuaries. It 



