2o6 PHYSIOLOGY OF THE INVERTEBRATA. 



network of vessels connected with the respiratory structures, 

 into a dorsal vessel, and thence by a connecting branch to 

 the posterior end of the heart. After a certain period, the 

 wave of contraction through the heart, and the course of the 

 blood, are generally reversed in direction ; and the blood 

 now flows from the ventral heart into the dorsal vessel, down 

 through the branching network into the abdominal or ven- 

 tral vessel, and so to the anterior end of the heart." 



The blood consists of a clear plasma containing colourless 

 corpuscles. 



In AiiipendicidarKi flahellnm. Professor Huxley states that 

 there are no corpuscles, and " the direction of the pulsations 

 of the heart is not reversed at intervals, as it is in the 

 Ascidians in general. M. Fol,* however, states that, in other 

 A'ppcndicularicc the reversal of the contractions of the heart 



takes place There are no distinct vessels, but the 



colourless fluid which takes the place of blood makes its way 

 through the interspaces between the ectoderm and^endoderm 

 and the various viscera." 



Concerning the velocity of the circulation in the Inverte- 

 hrata very little is known ; but it may be stated that the 

 blood in these animals is animated by a much slower move- 

 ment of translation than occurs in the Vcrtcbratct. 



* Etudes sur les ApiMHdicuhures, 1872. 



