CHAPTER I 



INTRODUCTION — POSITION OF MOLLUSCA IN THE ANIMAL KING- 

 DOM — CLASSIFICATION — ORIGIN OF LAND AND FRESH- 

 WATER MOLLUSCA 



It is the generally accepted opinion among men of science 

 that all life originated in the sea. Not that all parts of the sea 

 are equally favourable to the development of forms of life. The 

 ocean surface, with its entire absence of shelter or resting-place, 

 and the deep sea, whose abysses are always dark and cold and 

 changeless, offer little encouragement to plant or animal life, as 

 an original starting-point. True, both the surface and the depths 

 of the sea have become colonised by myriads of forms, MoUusca 

 amongst them, but these quarters are in the truest sense colo- 

 nised, for the ancestors of those who inhabit them in all proba- 

 bility migrated from elsewhere. 



It was no doubt the littoral region and the shallow waters 

 immediately below it, a region of changeable currents, of light 

 and shade, of variation, within definite limits, of temperature 

 and tide effects, which became the scene of the original develop- 

 ment of plant life, in other words, of the food-supply which 

 rendered possible its colonisation by higher animals. But the 

 'ittoral region, besides the advantages of tenancy which it offers 

 . > ariimal life, has also its drawbacks. The violence of the surf 

 nay beat its inhabitants in pieces, the retreat of the tide exposes 

 them, not merely to innumerable enemies in the shape of pre- 

 datory birds and beasts, but also to a change in the atmospheric 

 medium by which they are surrounded. Hence, in all proba- 

 bility, have arisen the various forms of adaptation which are 

 calcu.ctted to bring about the ' survival of the fittest ' ; hence, to 



VOL. Ill I B 



