MOLLUSCS 407 



by flapping their shells, but according to some highly competent 

 observers masses of them remain passive on the surface of the 

 mud with their long byssus threads waving about in the water. 

 It will probably be found that their behaviour varies in different 

 species. In any case the Glochidium's chance of further develop- 

 ment entirely depends upon whether its sticky thread happens 

 to touch the skin or gills of a fish. If this piece of good for- 

 tune should come about, it attaches itself firmly to the fish by 

 means of the hooks on the shell. The irritation thus set up 

 causes the skin or gill to grow out into a little capsule or cyst 

 enclosing the Glochidium, and within this shelter development is 

 continued. After a time, varying from a few weeks to several 

 months, the form of the adult is fully acquired, and the little 

 mussel falls out of the cyst into the mud. While 

 attached to the fish it is provided not merely with 

 shelter, but also with food, for it absorbs the juices 

 of its unfortunate host. In Anodonta the hooks 

 on the shell of the Glochidium are very well de- 



i i 1 1 i i r i e- i Fig. 929. Glochidia 



veloped, and in this case the skin of the fish is O f Freshwater Mussel 



attacked. But in Unio the hooks are much 

 weaker, and fix themselves to the softer gills 

 (fig. 929). The remarkable life-history just outlined is evidently 

 an adaptation to freshwater conditions, for it prevents the delicate 

 larvae from being swept down to the sea, where they would perish, 

 and is also important as regards distribution of the species. Some 

 freshwater bivalves, such as Cyclas and Pisidium, gain the former 

 end by suppressing the larval stage altogether, developing directly 

 into the adult form by a series of gradual changes. 



Dwellings of Bivalve Molluscs. Most of these creatures live 

 in mud or sand, through which they slowly plough their way ; but 

 some dwell in holes or crevices, others bore in various hard 

 substances, and others again attach themselves to stones, &c., 

 or construct a home from foreign bodies. 



Attachment to firm objects is often effected by means of silky 

 threads making up what is known as a byssus, springing from the 

 back of the foot near its base. A well-known example is afforded 

 by the Edible Mussel (Mytilus edulis, fig. 930), of which great 

 numbers may often be seen adhering by their black threads to 

 rocks or piles near low- water mark. But this animal is not neces- 

 sarily doomed to spend the whole of its adult life glued to the 



