244 THE HEART. 



I am afraid my attempt to describe these phenomena 

 is but partially successful : I am sure it cannot convey 

 to you any adequate idea of the spectacle itself. Have 

 you ever gazed with interest on a complicated piece 

 of machinery in motion, such as is common in our 

 large manufacturing houses ? If so, I dare say you 

 have felt a sort of pleased bewilderment at the multi- 

 tude of wheels and bands, rolling and circling in 

 incessant play, yet with the most perfect steadiness 

 and regularity. Something of that sort of impression 

 was made on my mind by the sight of the respiratory 

 organ of this tiny Ascidia, coupled as it Avas with 

 another simultaneous, equally extensive system of 

 movements, yet quite independent, and in nowise 

 interfering with the former. I mean the circulation 

 of the blood. 



At the very 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, of the appearance of which I 

 can give you a notion only by comparing it to 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 ; aud within it are seen 

 many minute colourless globules, floating freely in a 

 subtle fluid; this is the nutrient juice of the body, 

 which we may, without much violence, designate the 

 blood. Now see the circulation 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 onward 

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



