TUNIC AT A. 273 



studied in the living animal. A very singular condition of the circu- 

 lating system has thus been detected. The blood actually moves 

 backwards and forwards, to and from the heart in the same vessels, 

 as it was supposed to ebb and flow in the human veins before Harvey's 

 great discovery. The oscillation of the currents is not constant and 

 regular ; the blood is received from the vessel at one end of the heart, 

 and propelled by a contractile wave into the vessel at the opposite 

 end : after a true circulation has gone on in this course for a cer- 

 tain period, a change is observed in the course of the peristaltic 

 contractions of the heart; the blood for an instant stagnates in 

 the vessels, and then the wave travels in the opposite direction ; 

 the heart drives the blood into the vessel from which it had before 

 received it, and the course of the circulation is reversed. In the 

 compound Ascidians the vascular systems of the different individuals 

 anastomose freely with each other. 



At first sight it is difficult to conceive how the fixed and compound 

 Ascidians can multiply their race in situations at a distance from 

 that which they themselves occupy. This difficulty has been removed 

 by M. M. Audouin and Milne Edwards, who observed that the young 

 of the compound Ascidians were not only at their origin solitary and 

 free, but possessed the power of swimming rapidly by the aid of 

 the undulatory movements of a long tail. They were seen occasionally 

 to attach themselves to the side of the vessel of sea-water containing 

 them, and then to recommence their course, as if to seek a more 

 suitable point of attachment. After two days of free and locomotive 

 life, they finally fixed themselves; and, when detached, remained 

 motionless. 



These phenomena are now known to be common to the embryo of 

 many of the lower sedentary animals. In regard to the Ascidians, it 

 has been confirmed by M. Sars in the Botrylli of the coast of Nor- 

 way, and has been more recently observed by Sir John Graham 

 Dalyell, in a solitary Ascidian of the Frith of Forth. 



In the genera Polyclinum and Amaroucium, Dr. Edwards has ob- 

 served that the ovum, whilst still included in the ovarian mass, con- 

 sists of the small central germinal vesicle, of a granular vitellus and a 

 vitelline membrane. In the progress of the ovum to the cloacal 

 cavity, the yolk acquires a deep yellow colour, the germinal vesicle 

 disappears, and in its place there is a nebulous speck upon the surface 

 of the yolk. This is doubtless the remains of the germinal vesicle, 

 which has come to the surface of the yolk to meet the impregnating 

 influence, and has undergone the changes by fissiparous multipli- 

 cation, to which I have so often had occasion to allude. Dr. Ed- 



T 



