170 THE VITAL FUNCTIONS. 



Chara. Its general course was parallel to the slightly spiral 

 lines of irregular spots on the surface of the tube, ascending 

 on the one side, and descending on the other; each of the 

 opposite currents occu])ying one-half of the circumference 

 of the cylindric cavity. At the knots, or contracted parts 

 of the tube, sliglit eihlies were noticed in the currents; and 

 at each end of the tube the particles were seen to turn round, 

 and pass over to the otiier side. In various species of *S'er- 

 tiilariic the stream does not flow in the same constant di- 

 rection; but, after a time, its velocity is retarded, and it 

 then either stops, or exhibits irregular eddies, previous to 

 its return in an op])osite course; and so on alternately, like 

 the ebb and flow of the tide. If the currents be designedly 

 obstructed in any part of the stem, those in the branches 

 go on without inteiTuptlon, and independent!}^ of the rest. 

 The most remarkable circumstance attend In 2; these streams 

 of fluid is that they appear to traverse the cavity of the sto- 

 mach itself, flowing from the axis of the stem into that or- 

 gan, and returning into the stem without any visible cause 

 determining these movements. Similar phenomena were 

 observed by Mr. Lister in Campanxdarhe and Flumula- 

 rix. 



In some of the minuter species of Crustacea the fluids 

 have been seen, by the aid of the microscope, moving with- 

 in the cavities of the body, as if by a spontaneous impulse, 

 without the aid of a propelling organ, and apparently with- ' 

 out being confined in membranous channels, or tubes of any 

 sort. This kind of diff'used circulation is also seen in the 

 embryos of various animals, at the earliest periods of their 

 development, and before any vessels are formed. 



§ 2. Vascular Circulation. 



The next step in the gradation of structures consists in 

 the presence of vessels, within which the fluids arc confined, 

 and by which their course and their velocity are regulated j 



