44 INTRODUCTION. 



with mucus or with a sebaceous matter, is in many, provided 

 with an epidemics, hairs, horny scales, or a calcareous crust, 

 and becomes also an organ of defence and support. In this 

 case, some parts are not covered with these envelopes, are 

 very moveabie,and constitute particular organs of touch; such 

 are the tentacula of sea-urchins, those of some fish, those 

 of mollusca; the antennae of insects and Crustacea, &c. 



The organ of taste is not to be met with, distinctly, in all 

 animals which digest, and )~et it seems requisite that this sen- 

 sation should exist in all. In the radiated animals, nothing is 

 to be perceived, at the entrance of the alimentary canal, which 

 seems to be that organ. The same is the case with respect to 

 the mollusca and articulated animals. In some insects, how- 

 ever, this faculty is supposed to exist in the extremity of the 

 proboscis or palpi; finally, all the vertebrata are far from hav- 

 ing a tongue organized in a manner to enable them to taste. 



The organ of smell seems to be wanting in a great number 

 of animals; insects, however, Crustacea, and arachnides are sen- 

 sible to odours, but the precise seat of their sensation is en- 

 tirely unknown. The same may be said with respect to the 

 mollusca. Even in the vertebrata the nasal fossae do not tra- 

 verse the face in all the classes. 



The organ of hearing or the ear is not found in the lower 

 classes of animals, and sound seems to be only perceived as a 

 tactile impression. Among articulated animals the crab is the 

 only one in which we observe an ear, though they all hear 

 very well. The ear in the crab consist of a bag filled with a 

 gelatinous lymph, which receives a separate nerve. In the 

 same manner, the mollusca and cephalopoda have this organ, 

 which exists in the vertebrata, and which presents a great va- 

 riety. 



Light has also an action on the skin of all animals, and on 

 every part exposed to its influence; but the faculty of sight is 

 possessed only by those who have its organ, the eye. The 

 radiated animals have no eyes. A portion of the annelides are 

 deprived of it; in the others we only meet with the rudiments; 

 it is a mere black spot. The articulata with feet, viz: the crus- 

 tacea, arachnides and insecta, have all eyes which may be of two 



