CIRCULATION OF THE BLOOD IN PYROSOMA. 277 



if by pressure or accident the contents of the nurse happen to 

 escape, the heart may remain attached to the internal surface 

 of the external layer, as I have chanced to see it. 



In this case the whole presents a very irregular aspect, 

 similar to that which has been figured by Professor Vogt.' 

 More strictly, this heart is placed in the space between the 

 external and internal tunics of the nurse, which space I shall 

 term " the peripheral sinus." The heart beats vigorously ; 

 indeed, it is almost impossible to see it when it is not beating. 

 The pulsations are similar to those of the heart of the 

 Salpse, Ascidians in general, and of the adult Pyrosoma ; 

 that is to say, the heart begins with a movement of consider- 

 able energy at one of its extremities, which extends in a con- 

 tinuous undulation throughout its length ; the movement is, 

 in fact, peristaltic, like that of the intestine of the higher 

 animals. 



Scarcely is a complete pulsation finished when a second 

 commences, then a third, and so on. But after 37, 39, 40, 

 or 50 of these pulsations, the last of which are very uncer- 

 tain, the heart makes a brief pause, and then recommences ; 

 but now the movement is changed, the direction of the con- 

 traction is reversed. 



This heart has an importance and a signification greater 

 than might be supposed, since from it the cord takes its 

 origin which unites the four embryos ; and since I have seen 

 that the latter is vascular, the heart appears to be the centre 

 of a " social " vascular apparatus common to all the embryos 

 and to the nurse. 



From the extract from Professor Huxley's memoir, which 

 is all I have been able to consult, it is not possible to tell if 

 he considered the cord to be a canal open at its ends, when 

 he says that the primitive cord of the blastoderm, which 

 unites the embryos, becomes converted at last into *' a long 

 canal." It is, however, certain that he would not have made 

 out this canal to be vascular, because he studied the matter 

 in specimens of Pyrosoma preserved in alcohol. But it is, in 

 fact, so, and is, moreover, composed of two contiguous and 

 parallel vessels, one of which arises directly from the heart, 

 whilst the other communicates with the peripheral sinus of 

 the nurse, and then forms a system of blood-vessels in the 

 embryos by giving origin to branches. 



In this way I was led to inquire into the origin and dis- 

 tribution of this vascular system. 



In the first stage of the development of the embryos, that 



^ " lleclierches sur les Animaux infer, de la Mediterranee,' 1S53, ii nicm.j 

 tub. X, fig. 10. 



