46 EVENINGS AT THE MICROSCOPE 



veins; but the smaller streamlets flow in any direction, 

 and frequently send out side- branches, which presently 

 return into the stream from which they issued, or unite 

 with others in a very irregular network. These are the 

 capillaries, which feed the veins, and which are them- 

 selves fed by the arteries, whose course is in the opposite 

 direction; viz., from the body. These, however, are with 

 difficulty seen: they are more deeply seated in the tissues, 

 and are less spread over the webs, being generally placed 

 along the borders of the toes; they are, moreover, fewer 

 and smaller than the veins ; but the blood in them usually 

 flows with more impetuous rapidity. 



The variations in the impetus of the current which we 

 observe in the same vessel are probably owing to the men- 

 tal emotions of the animal; alarm at its unusual position, 

 and at the confinement which it feels when it endeavors 

 to move, may suspend the action of the heart, and thus 

 cause an interruption in the flow; or analogous emotions 

 may quicken the pulse. We will, however, now release 

 our little prisoner, who, though glad to be at liberty, is, 

 as you see, none the worse for his temporary imprison- 

 ment. 



Let us now look at the circulation of the blood in one 

 of the Invertebrate Animals. In this thin glass cell of 

 sea- water is a small fragment of sea- weed, and attached to 

 one of its slender filaments you may see three or four tiny 

 knobs of jelly, clustered together like a bunch of grapes. 

 These are animals; each endowed with a distinct life, but 

 associated together by a common stalk, which maintains 

 the mutual vital connection of the whole. It is one of the 

 Social Tunicata, and is named Perophora Listeri. 



Though each globose knob is no larger than a small 



