ANATOMY AND DEVELOPMENT. 193 



from one end of the heart to the other ; but the chief 

 peculiarity in connexion with it is, that after a certain 

 number of contractions in one direction the heart makes a 

 brief pause and then commences to contract again in the 

 opposite direction, and so it goes on contracting now in one 

 direction and now in the other. This phenomenon of 

 the periodic reversal of the direction of contraction of the 

 Tunicate heart is known as the recurrent action of the 

 heart, and was discovered in 1824 by VAN HASSELT. 

 The discovery was first made in the case of Salpa, but it 

 has since been found to hold good for all Tunicates. 



When the heart contracts from its posterior to its an- 

 terior extremity, that is to say, in the postero-anterior 

 direction, the blood is thereby propelled forwards into the 

 blood-sinus which lies below the endostyle, and from this it 

 passes into sinuses which run transversely into the bran- 

 chial bars. In the basket-work formed by the intercross- 

 ing of the branchial bars, the blood has a complicated 

 and irregular course, and is finally collected into the dorsal 

 sinus which lies above the dorsal lamina. Here it flows 

 backwards, and after passing in amongst the viscera arrives 

 back to the heart. (Other branches of the sinuses pass 

 into the test, where they end in curious knob-like dilata- 

 tions.) 



On the contrary, when the heart contracts in the reversed 

 or antero-posterior direction, the blood which has already 

 been oxygenated in its passage through the branchial bars 

 is sent to the viscera direct, and from there it collects 

 into the dorsal sinus, from which it is distributed over the 

 branchial sac, and so into the sub-endostylar or ventral 

 sinus, in which, it flows backwards to the heart. 



On account of the above peculiarities relating to its 

 independent origin, the histological structure of its wall, 



