GENERAL CHARACTERS OF BIVALVES. 303 



active forms like Pecten and Lima being exceptional. In. 

 the most sluggish, such as oysters, the foot is degenerate. 

 They feed on microscopic plants !\nd animals and on 

 particles of organic debris, which are wafted to the mouth 

 by the lashing cilia on the gills. The mouth is without any 

 prehensile or rasping structures. Bilateral symmetry is 

 retained. Thus there are two mantle folds, two shell-valves 

 united dorsally by an elastic ligament, two auricles to the 

 heart ; and the plate-like gills (ctenidia), formed from the 

 growing together of gill-filaments, — the kidneys (nephridia), 

 the digestive gland, and the reproductive organs are all 

 paired. But the head region remains undeveloped. There 

 are no head-eyes. The nervous system usually consists of 

 three pairs of ganglia — {a) in the head (cerebro-pleurals), {b) 

 in the foot (pedals), and \c) at the posterior end of the body 

 (viscerals). The sexes are usually separate, but there are 

 many bisexual forms. The reproductive organs (ovaries and 

 testes) are simple, without accessory structures. 



Further Remarks on Bivalves in General. — We may associate the 

 sluggish habits and sedentary life of bivalves, ( I ) with the undeveloped 

 state of the head-region, (2) with the largeness of the plate-like gills 

 which waft food-particles to the mouth, and (3) with the thick limy shells. 

 We may reasonably associate these and other facts of structure (e.g., 

 the absence of head-eyes, biting or rasping organs) with the conditions 

 of life, without being able to say very precisely what the relation is. It 

 seems to me most likely that sluggish habits have cumulative and 

 manifold results in the course of generations, and that the structural 

 changes produced by surroundings, or by use and disuse of parts, have 

 constitutional consequences which may affect the germ-cells, that is to 

 say, the offspring. To others the adaptations seem to be most readily 

 explained as the result of the natural selection of fortuitous or indefinite 

 variations. In thinking about the sluggishness of most bivalves, we 

 must not of course forget that the larval trochospheres and veligers are 

 very active, perhaps almost too active, young creatures. 



Habit. — Most bivalves, as every one knows, live in the sea, and extend 

 from the sand of the shore to great depths. They occur in all parts of 

 the world, though only a few forms like the edible mussel {Mytilus 

 edulis) can be called cosmopolitan. Some, such as oysters, can be 

 accustomed to brackish water. The fresh-water forms may have found 

 their habitat 'in two ways — (a) a few may have crept slowly up from 

 estuary to river, from river to lake ; Dreissena polyviorpha has been 

 carried on the bottom of ships from the Black Sea to the rivers and 

 canals of Northern Europe ; and it is likely that aquatic birds have 

 assisted in distributing little bivalves like Cyclas ; (6) on the other hand, 

 it is more likely that the fresh-water mussels (Unio, Anodonta, etc.). 



