CIRCULATION IN THE TUNICATA. 215 



One of these vessels receives, as it is believed, all the blood 

 from the branchiae, and is in consequence named the bran- 

 chial vein ; the other, of greater length, is an aorta to 

 distribute the blood through the whole system.* 



" In the Compound Ascidia?, Mr. Lister has recently dis- 

 covered one of the most remarkable modifications of the 

 circulation with which we are acquainted. Mr. Lister finds 

 that the different Ascidiae of a branched animal are not only 

 connected together by the polypi ferous stem, but have a 

 common circulation. In each individual there is a heart 

 consisting of one cavity only, and pulsating about thirty or 

 forty times in a minute. In the common stem, the motion 

 of the globules of the blood indicates distinctly two currents 

 running in opposite directions. One of the currents enters 

 the Ascidia by its peduncle and proceeds directly to the 

 heart ; the blood issuing from the heart is propelled into the 

 gills as well as the system at once, and upon its return from 

 thence the returning current proceeds out of the animal by 

 its peduncle again into the common stem, whence it goes 

 to circulate through another of the Ascidise attached to the 

 stem. The directions of the currents appeared to be re- 

 versed every two minutes or less. A ccording to Mr. Lister, 

 when one of the Ascidiae is separated from the common stem, 

 its circulation goes on in an independent manner, the blood 

 returning from the body being conducted into the heart; but 

 the alternation of the directions still continues, — a circum- 

 stance which points out an important difference between the 

 Compound and the Simple Ascidias, in which last the circu- 

 lating fluid is generally believed to pass from the gills into 

 the heart, and to hold continually the same direction." f 



These observations of Mr. Lister apply to the Social, and 

 not to the Tunicata properly called Compound. Milne-Ed- 

 wards discovered several notable peculiarities in the latter, 

 the sum of which is, that the heart does not contract all at 

 once, as it does in other animals, but in a manner which is 

 comparable rather to the peristaltic motions of the intestines 

 in the vertebrates. Beginning at one end of the heart, the 

 contraction is propagated, in an undulatory manner, to the 

 opposite extremity ; and after a time, and a momentary 

 pause, the order of the contraction is reversed, and the 

 motion proceeds in the contrary direction. The flow of the 

 blood obeys of course this alternate action in its moving 



* Savigny Mem. sur lcs Anim. s. Vert. ii. 113. 



t A. Thomson in Cyclop. Anat. and Phy. i. 650. Mr. Lister has given 

 an account of his observations at greater length in the Phil. Trans, for 1834, 

 p. 380. 



