240 



BY THE DEEP SEA. 



elaborate system of ventricles and auricles, with their regula- 

 ting valves. The Ascidian's heart is simply a tube open at 

 each end, and by its steady pulsation that is, its alternate 

 contraction and expansion, it sets the blood flowing to the 

 blood-vessels that line the walls of the branchial cavity, where 

 it absorbs oxygen from the continuous flow of fresh sea-water 

 that passes through it. When this end has been attained, a 

 curious and unique " reversal of the engine " takes place : 



Ascidia mentula. 



CURRANT-SQUIRTER (STYELA GROSSULARIA). 



there is such an opposite action of the heart, that all this 

 vivified blood is withdrawn from the neighbourhood of the 

 Branchial chamber and sent flowing to remote parts of the 

 body. 



