SPIDERS AND MITES 251 - 



uniformity in the direction of the pulsatory waves; some- 

 times, as in the limb we are watching, they proceed from 

 the body toward the extremity; but, in some of the others, 

 we shall probably find, even at the same moment, that the 

 waves have a retrograde course; and this contrariety may 

 occur in two contiguous limbs on the same side of the ani- 

 mal. By continuing our observation for some minutes, we 

 shall find also that its force is varying and uncertain; 

 strong and regular at one time, weak and vacillating at 

 another, and sometimes even quite intermitted, or, at least, 

 quite imperceptible. 



By selecting a limb in which the movements are strong, 

 you may trace the vessel to its termination in a blind sac 

 in the last joint but one of the limb; and then follow it 

 up to its junction with a great vessel which runs longitudi- 

 nally through the trunk, of which all the vessels that per- 

 meate the limbs are branches, and whence the circulating 

 globules all proceed. This great vessel is the stomach and 

 this circulation is the provision for dispersing the nutritive 

 properties of the food to all parts of the system. There is 

 in these humble and simply organized animals no proper 

 blood, or, at least, none included in a system of arteries 

 and veins ; but the products of digestion are carried to the 

 most distant parts of the body, through this extraordinary 

 development of the stomach or intestine (both in one), and 

 through this series of blind canals, by means of their own 

 irregular contractions, aided by the muscular movements 

 of the body and limbs. 



You would scarcely forgive me if I took no opportu- 

 nity of showing you the Cheese-mite, that first object of 

 wonder to every child that looks through a magnifying 

 glass. And no place so suitable for its introduction, as in 



