84 EVENINGS AT THE MICROSCOPE. 



foot-sta]ks from the sister globules, and all together are 

 attached to the sea-weed. Each foot-stalk has an organic 

 core, into which a vessel passes from the body. 



Your attention is first arrested by the breathing sac, 

 with its rows of oblong cells, all in wheel-like motion. It 

 is indeed a wonderful object ; but for the present neglect 

 this, as we will return to it shortly, and direct your consi- 

 deration to the course of the blood. 



It is true the fluid which I so name is not red, like 

 that of the Frog whioh you have just been gazing at, nor 

 does it carry disks of the same elegantly regular form. 

 But you have the advantage here of tracing, at one view, 

 the whole course of the circulation, from its first rush out 

 of the heart, to its return into that organ again. 



At the bottom of the interior, below the breathing sac, 

 there is an oblong cavity, through whose centre there runs 

 a long transparent vessel, formed of a delicate membrane, 

 the appearance of which resembles that of a long bag, 

 pointed (but not closed) at either end, and then twisted in 

 some unintelligible manner, so as to make three turns. 

 This is the heart; and within it are seen many minute 

 colourless globules, floating freely in a subtle fluid : this is 

 the nourishing juice of the body, which we may, without 

 much violence, designate the blood. Now see the circula- 

 tion of this fluid. The membranous bag gives a spasmodic 

 contraction at one end, and drives forward the globules 

 contained there ; the contraction in an instant passes on- 

 ward along the three twists of the heart (the part behind 

 expanding immediately as the action passes on), and the 

 globules are forcibly expelled through the narrow but open 

 extremity. Meanwhile, globules from around the other 

 end have rushed in as soon as that part resumed its usual 

 width, which in turn are driven forward by a periodic repe- 

 tition of the contraction and expansion of the heart. 



The globules thus periodically driven forth from the 

 heart, now let us watch, and see what becomes of them. 



