Inter-Relations of Living Creatures 640 



linkage between freshwater mussels and minnows. The eggs of 

 the freshwater mussel are produced about midsummer in Britain, 

 but they are not set free. They develop in a special brood- 

 chamber, the cavity of the basketwork-like outer gill. They turn 

 into pin-head bivalve larva? called Glochidia, which are not allowed 

 to escape till early in the following year. Moreover, they are not 

 liberated unless a fish such as a minnow comes swimming slowly 

 past. When the larvae are set free they swim in the water clap- 

 ping their valves together, exuding delicate gluey threads, and 

 making for the fish. Some lucky ones get attached to the min- 

 now and burrow a little into the flesh. Here they undergo a great 

 change and eventually drop off into the mud, often far from where 

 they were born. It is very striking to find that a Continental 

 fish, the bitterling (Rhodeus amarus) , uses a long egg-laying tube 

 to inject its eggs into the freshwater mussel. The eggs develop 

 in the mussel's gill-chamber, and the larval bitterlings spend some 

 time there before they find their way out. So the freshwater 

 mussel is dependent on some fish, and the bitterling is dependent 

 on the freshwater mussel ! This is what is meant by linkage, and 

 scores of striking instances will be found in books that deal with 

 life-histories. 



One Creature on Another 



It often happens, especially in a crowded area such as the 

 sea-shore, that one animal settles down on the back of another, 

 as rock-barnacles on a crab's shell or a tube-inhabiting worm on a 

 buckie. This mode of life is called epizoic, and it may be adopted 

 by plants as well as by animals. Thus seaweeds are often at- 

 tached to crabs and even to aged lobsters, and a green Alga grows 

 on the shaggy hair of the tree-sloth. It may be an advantage to 

 the epizoic animals or plants to be carried about by an active 

 bearer. To the bearer it is probably in some cases a burden ; but 

 it is often quite indifferent; occasionally, as has been noticed in 

 the article on Disguise, it has a camouflaging utility. 



