CIRCULATION OF THE BLOOD IN PYROSOMA. 279 



its course in those embryos which have attained a higher 

 degree of development, as in this case one can clearly see 

 the peripheral sinus of the nurse. One of the vessels of the 

 cord — that which is directly continuous with the heart — after 

 a course of some length, is attached to the first embryo just 

 a little below the ganglion ; it penetrates the external tunic, 

 and is directed to the mouth ; is gently curved to the left, 

 and then continues to the endostyle ; it then passes beloAV 

 this, and descends with it to its extremity on the opposite 

 surface of the embryo. Here it breaks up into two parts, 

 one of which passes out of the external tunic, and forms one 

 of the vessels of the cord, which unites the first embryo to 

 the second; and the other, continuing its course in the 

 first embryo, arrives on the neural surface ; it passes out of 

 the external tunic near the ganglion, and becomes united to 

 that vessel, which came from the nurse, accompanies it in its 

 course, and opens into the peripheral sinus. This vessel of 

 the cord, which was given off" from the first embryo, at the 

 end of the endostyle, arrives at the second, penetrates the 

 external tunic below the ganglion, and forms similarly a 

 circle, having first given off near the inferior extremity of the 

 endostyle a vessel to the third embryo, and to the first the 

 other vessel of the cord which unites the two. In the third 

 embryo there is the same distribution of vessels, except that, 

 as in the second, there is an illusive change of aspect ; and 

 it is difficult to follow them, on account of their lying in 

 different planes. At last the cord arrives at the fourth em- 

 bryo, situated in the same plane as the first, and in this also 

 the vessels follow the same disposition, but at the end of the 

 endostyle the circle is' closed ; thus, the single original vas- 

 cular loop of the rudiments of the embryos, when further 

 developed, constitutes the entire vascular system. 



It is a very interesting spectacle to follow with the micro- 

 scope tlie rapid circulation of the blood in the entire system of 

 the embryos. The heart of the nurse beats, pushes the blood 

 into the first embryo by means of that vessel of the cord 

 with which it communicates ; the blood, after having here 

 made a complete round, returns to the nurse, and is emiDtied 

 into the peripheral sinus by means of the other vessels of 

 the cord. The same pulsations of the heart draw the blood 

 from this latter vessel, and oblige it to circulate in the peri- 

 pheral sinus and to enter anew the heart by its open extre- 

 mity. Since the heart closes the space between the internal 

 and external strata of the nurse, the blood cannot pass in 

 that direction, and must make the round ; but the greater 

 part of the blood passes from the first to tlie second embryo. 



