154 THE HARMONIES OF NATURE. 



The eyes, in those species where they have been detected, 

 appear among the fringing processes of the margin as a number 

 of glittering studs of metallic brilliancy, and though of a very 

 simple structure, no doubt give their owner an agreeable 

 impression of light, and in many cases a timely warning of 

 approaching danger. 



The whole construction, and generally the extremely re- 

 stricted locomotion of the bivalves, tells us at once that they 

 are unable to attack their prey, but must be satisfied with the 

 food which the sea-currents carry to their mouth. Yet they 

 have as little reason to complain as the equally slow or sessile 

 polyps, bryozoa, and tunicata, for the waters of the ocean harbour 

 such incalculable multitudes of microscopic animals and plants 

 that their moderate appetite never remains long unsatisfied. 



The same streams which aerate their blood also convey to 

 their mouth all the food which they require. 



The gasteropoda or snails are more highly organized than 

 the bivalve molluscs. The body is more symmetrical, there is 

 a greater distinction of parts, an obvious head, an evident tail ; 

 and, save that the body is without legs, we have often a con- 

 siderable outward resemblance to some vertebrate animal, in 

 the form of the body and in the expression of the countenance. 

 For here is a well-formed face, surmounted by two, four, or six 

 tentacula, commonly called horns, which either, as in the snail, 

 carry each an eye at its summit, or, as is the case in most of the 

 marine kinds, have an eye on a prominence at the base. But 

 very few remain firmly attached to some foreign body, and 

 make use of their rudimentary foot for no other purpose than 

 for opening or closing the lid of their shell ; generally a strong 

 muscular disk attached to the ventral surface of the body 

 (whence the whole order has derived its name of gasteropoda 

 or stomach-footers) serves either as an instrument by means of 

 which the animal can crawl, or, in rarer instances, is compressed 

 into a muscular membrane, useful in swimming. Some of the 

 gasteropoda are naked, while the majority are covered with a 

 shell ; some are formed for a terrestrial, others for an aquatic 

 life ; some are predacious, others herbivorous ; and thus it may 

 easily be imagined that their organization must offer numerous 

 modifications to suit such various modes of life. 



Although a superficial view might lead us to believe that 



