358 EVENINGS AT THE MICROSCOPE 



of this class of animals, and also in that of Zoophytes, 

 they play an important part in the economy of the creat- 

 ure. I shall probably take occasion to exhibit them to 

 you under conditions more favorable than are presented 

 here; viz., in the Sea-anemones, where they attain far 

 greater dimensions; and therefore I will merely say here 

 that each one of these tiny vesicles carries a barbed and 

 poisoned arrow, which can be shot forth at the pleasure of 

 the animal with great force, and to an amazing length 

 that hundreds are usually shot together and that this is 

 the provision which the All- wise Glod has given to these 

 apparently helpless animals for securing and subduing 

 their prey. 



There is, however, another organ still more conspicu- 

 ous in our little tiarsia, of which I have not yet spoken. 

 As the whole animal has the most absolute transparency, 

 we see that the roof of the bell is much thicker than the 

 sides, and that it gradually thins off to the edge. The in- 

 terior surface is called the sub-umbrella, and it carries 

 within its substance four slender tubes, which, radiating 

 from the centre of the roof, proceed to the margin, where 

 they communicate with another similar canal which runs 

 round the circumference, sending off branches into the 

 tentacles. This is the circulatory system; and you may 

 see, with the magnifying power which you are at present 

 using, that a clear fluid is moving rapidly within all these 

 canals, carrying minute granules; not with an even for- 

 ward current, but with an irregular jerking vacillating 

 movement, as if several conflicting eddies were in the 

 stream. Yet we discern that, on the whole, the granules 

 are moved forward; passing from the centre of radiation 

 toward the margin, when we see them slip into the mar- 



