2 ARTICULATED ANIMALS. 



dilations to produce the same variety of motion. From this 

 also results a greater loss of force in the muscles, and conse- 

 quently more general weakness in each animal, in proportion 

 to its size. 



But the joints which compose the body have not always 

 this sort of articulation : they are most usually united merely 

 by flexible membranes, or emboxed one within the other ; 

 and then their motions are more varied, but do not possess 

 the same force. 



The system of organs on which the articulated animals are 

 most similar to each other, is that of the nerves. 



Their brain, situated on the oesophagus, and furnishing 

 nerves to the parts which adhere to the head, is very small. 

 Two cords, which embrace the cesophagus, are continued 

 along the belly, uniting, from space to space, into double 

 knots or ganglia, from which proceed the nerves of the body 

 and of the limbs. Each of these ganglia performs the 

 functions of a brain, for the surrounding parts, and suffices to 

 preserve their sensibility for a certain time, when the animal 

 has been divided. If we add to this, that the jaws of these 

 animals, when they have any, are always lateral, and move 

 from without to within, and not from top to bottom, and that 

 there has not yet been discovered in any of them a distinct 

 organ of smell, we shall have pretty nearly expressed all that 

 may be said of them in general. But the existence of the 

 organs of hearing ; the existence, number, and form of those 

 of sight ; the product, and the mode of generation *; the nature 

 of respiration ; the existence of the organs of circulation; and 

 even the colour of the blood, present great variations, which 

 it v/ill be necessary to study in the different sub-divisions. 



* A remarkable discovery on this subject is that of M. Herold, that in 

 the egg of the crastacea and arachnida, the vitellus communicates with the 

 interior by the back. See his dissertation on the egg of spiders, Marburg, 

 1824, and that of M. Rathke on the egg of the Astaci, Leipzig, 1829. 



