2 STRUCTURE OF MOLLUSCA, 



of creatures, chiefly aquatic, but some terrestrial ; 

 some capable of locomotion, others fixed to one spot 

 for life ; some inert and destitute of the organs 

 of vision, others furnished with eyes, and others with 

 arms for seizing their prey. Among all, such striking 

 organic differences, modes, and appearances, meet us, 

 that, on a superficial glance, the characters which bind 

 them into a common group would seem to be altogether 

 unsettled. 



When we look at the class of mammalia, or that of 

 birds, of fishes, or of reptiles, we at once perceive an 

 uniformity of the plan on which the creatures of each 

 class are modelled, and from w^hich they do not essen- 

 tially depart. The extent to which the farthest from the 

 standard diverges is limited ; the intermediate links con- 

 duct us to it, and we merely behold in it a modification 

 of the common type of structure. Still further, if we 

 consider mammalia, birds, reptiles, and fishes, to be 

 comprehended, as they are, under one sub-kingdom, 

 embracing their respective classes, we shall then find 

 that they all agree in certain leading characteristics, 

 which are never failing, however great the variation 

 may be in the form and modifications of other parts 

 of the structure. For example, all have an internal 



