d 



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, Mollusca 

 amongst them, but these quarters are in the truest sense colonised, 

 for the ancestors of those who inhabit them in all probability 

 migrated from elsewhere. 



It was no doubt the littoral region and the shallow waters 

 immediately below it, a region of changeable cuiTents, 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 

 littoral region, besides the advantages of tenancy which it offers 

 to animal life, has also its drawbacks. The violence of the sm^f 

 may 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 ^^arious forms of adaptation which are 

 cakndated to bring about the 'survival of the fittest'; hence, to 



VOL. Ill 5 B 



