28.2 PAVESI ON CIRCULATION OF THE BLOOD IN PYROSOMA. 



vessel which passes from the ganglion towards the mouth, 

 turns round there to the left, and then passes along the endo- 

 style, exactly as in the compound embryos, but I cannot say 

 any more, because I have not had a favorable opportunity of 

 observing the circulation in this form of embryo. However, 

 we may conclude with certainty that it must take place in the 

 same way as in the compound embryos. 



It remains, finally, to consider how the circulation is 

 carried on in the adult Ascidians, and here, for all that I have 

 seen, I completely agree with the illustrious naturalists cited 

 above. The heart lies at the extremity of the endostyle, has 

 the ordinary form of a tube, and is enclosed in a pericardium. 

 Its pulsations are alternate in the direction of their move- 

 ment, variable in their duration, and interrupted by pauses 

 of greater or less length. In one case I obtained the follow- 

 ing numerical results : — 34 pulsations towards the cloaca, 

 then a very brief pause, followed by 20 pulsations towards the 

 mouth ; after these, S6, 12, 84, 13, interrupted by varying 

 pauses and followed by uncertain oscillations, with three 

 pulsations every five seconds ; then 36 towards the mouth, 

 20 in the opposite direction, 28 again forwards, and so on. 

 In another case 20 pulsations towards the mouth, then a 

 pause for several seconds, followed by 43 pulsations directed 

 posteriorly, intercalated by uncertain pulsations in one direc- 

 tion or the other. 



The space between the two tunics of the Ascidian is 

 occupied by the blood ; it forms always the middle or vascular 

 stratum, and presents the form of a great sinus, in which are 

 found the ganglion, and in the superior part certain unde- 

 termined cellular organs ; these are phosphorescent organs of 

 M. Panceri. I cannot say that the blood passes behind them, 

 since they are attached to the external stratum, and the sinus 

 is not closed here, as Mr. Huxley thinks. 



Below, the sinus expands in correspondence with the 

 organs of digestion, which are immersed in it, and in the 

 lateral sinuses which surround the ovary and the testicles. 

 The branchire are all traversed by the blood in their trans- 

 verse canals, between the clefts which are limited by 

 nucleated, ciliated cells, being themselves attached to the 

 internal tunic by their anterior and posterior surface, and 

 communicating in consequence with the great sinus. 



In all cases the globules of the blood are very numerous, 

 round, and of about -007 mm. in diameter. The external 

 tunic of the vessels of the embryos is formed by a simple 

 stratum of muscular fibre-cells, with very obvious nuclei. 



In conclusion, it may be stated that — 1. The circulation 



