INTRODUCTION. XVU 



bouring parts. The sexes are in some united in 

 the same individual, in others separate. They 

 multiply by eggs. 



The Molluscs have the organs of digestion and 

 circulation well developed ; a liver, stomach, in- 

 testines, a heart with two chambers, arteries and 

 veins circulating cold blood ; a nodulated nervous 

 system, organs of touch, rudiments of a tongue, 

 and something like an organ of hearing, and a 

 respiratory system. The organs of locomotion 

 are not much developed. 



The Crustacea, or such animals as resemble the 

 lobster, possess lateral appendages fixed to the 

 trunk, which assist them to move : their structure 

 is similar to that of the molluscae ; but they have, 

 in addition, a more perfect apparatus of the senses. 



Ascending in the scale of beings, we next come to 



INSECTS. 



The English word insect is derived from the 

 Latin word insectum, which is probably a corrup- 

 tion or contraction of intersectum, " cut between ;" 

 and the name as applied to a class of animals, is 

 doubtlessly suggested by the bodies of these ani- 

 mals being so made up of distinct parts as to give 

 the appearance of their being notched or inter- 

 sected. 



Insects have organs of nutrition, locomotion, 

 generation, and sensation. Their organization is 

 defective principally in the circulating and respi- 

 ratory systems. They inhabit the earth, the air, 

 or the water, and move with rapidity in all situa- 

 B2 



