TUNICATA. 477 



backwards and forwards, to and from the heart in the same ve.^sels, 

 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 

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

 the heart di'ives 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. The veins are in the condition 

 of large lacunar sinuses. The heart and vessels circulate blood, not 

 water: if the vessels, as some contend, had no proper tunics (and 

 their transparency in the living Ascidians renders them, in most of 

 the sinuses, invisible), the sea-water which freely passes from the 

 branchial to the muscular cavities would flow into the so-called inter- 

 visceral lacuniK, were they merely such as they seem. 



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 re- 

 moved by MM. 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 de- 

 tached, 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 Sars in the Botrylli of the coast of Nor- 

 way * ; by Sir John Graham Dalyell, in a solitary Ascidian of the 

 Frith of Forth; and the embryogeny of the Cynthia ampulla has been 

 well followed out by Prof. Van Beneden.f The solitary Ascidiae are 

 of distinct sex. In the male a generative gland {Jig. 179, k), generally 

 dendritic in shape, occupies the concavity of the intestinal fold, and 

 sends a short and simple duct to terminate near the anus. In the 

 female of the Cynthia tuberculata there are two ramified ovaria ; 

 the ovisacs being appended to the branches of a central stem, passing 



* CCXCVI. t CCXCVII. 



