OF ANIMALS. 43 



to be homogeneous, we perceive no particular organ for mo- 

 tion, and nevertheless these infusory animalcule move about 

 with great rapidity. There are other animals a little more com- 

 plicated, which are yet unprovided with any kind of distinct 

 muscular organ: such as the rotiferae, which have a particular 

 rotatory organ, or like the polypi, which have around their 

 mouth tentacula, the movement of which agitates the water, 

 and with which they attract and seize nutritious substances, 

 and some of which possess, besides, movements performed by 

 the whole body. The proper organ of visible motion, the 

 muscular fibre, exists in the acalepha, and in the echinoder- 

 mata, the muscular system of which is supported by a well or- 

 ganized skin, and in all the more elevated animals in which 

 the apparent movements either general or partial, are pro- 

 duced by the action of these organs. The muscular fibres, in 

 all animals, which have any*, supply the external and internal 

 skin: they also form the heart of such animals as possess one. 

 Among animals, some have the skin as soft as jhe other parts 

 of the body; in a great many, it contains within its thickness 

 indurations, either calcarious, or horny, which shield the ani- 

 mal from external injuries, and which being moveable on each 

 other, transmit to the parts they support, the motion that they 

 have previously received from the muscles. In the verte- 

 brate animals, this latter office is fulfilled by moveable, articu- 

 lated, internal bones, and which for this reason are provided 

 with a great mass of muscles which is either wanting in the 

 invertebrata, or is attached on their cataphracted or indurated 

 skin. 



28. In the simplest animals, the organs of the sensations 

 have no distinct existence. The whole body seems to receive 

 impressions as it executes movements. In those which have 

 an external and internal skin different from the remaining 

 parts of the body, and all from the polypi upwards, have this 

 arrangement, the skin, besides the function of absorbing nu- 

 tritious substances, receives the impression of external bodies. 

 In those animals which have a very soft skin and but little dif- 

 ferent from the other parts, it is every way equally sensitive. 

 But the part of the skin which is moistened in various animals 



