34 INTRODUCTION. 



j which have besides a thorax, sometimes distinct from 

 the head and abdomen, and at others confounded with one or 

 both parts of the trunk. In the vertebrated animals the head 

 is always distinct, but the thorax is sometimes confounded 

 with the abdomen. The appendages present also different va- 

 rieties; in the infusoria there are small ones called cilias. 

 The radiated animals have the mouth surrounded with appen- 

 dages called tentacula, which are destined for motion and sen- 

 sation. The same is observed in some mollusca, which have 

 sensitive tentacula, and other fleshy productions, called arms 

 or feet, for the purpose of locomotion. The crustaceous ani- 

 mals and insects have antennae, articulated filaments, of very 

 diversified shapes affixed to the head, and which seem to be 

 organs of sensation. The same may be said of their palpi, 

 that are found also in the arachnida. The lateral appendages 

 are double, essentially intended for motion, and are called 

 limbs when they are articulated, the rudiments of them may 

 be observed in the cirrhopoda and in the setigerous an- 

 nelides; they are found in great number in the myriapoda; 

 they are also found in a considerable, but variable number, in 

 the Crustacea; there are eight in the arachnides, and six in the 

 true insecta, which possess, for the most part, either four or 

 two wings. In the vertebrata, there are never more than four 

 limbs. 



19. The organs of nutrition present a very great diver- 

 sity. In the most simple animals, the infusoria, this function 

 consists solely in an external absorption or imbibition, the ma- 

 terials of which penetrates every part of the body of the ani- 

 mal, and is immediately assimilated and afterwards excreted ; 

 this simplicity of organization is to be found in some intes- 

 tinal worms, and in some of the acalepha. 



In animals, a degree higher in the scale, we find an intesti- 

 nal cavity excavated in the substance of the body, and from 

 this moment absorption is performed by both surfaces, and 

 especially by the internal one. This simple cavity is observ- 

 ed in some polypi. At a still higher degree, this cavity con- 

 sists of a membranous sac, distinct from the mass of the body, 

 formed by a membrane or internal skin, continuous and analo- 



