TUNICATA : ALTERNATING DIRECTTON OF CIRCULATION. 585 



fissure makes two or three turns of a spiral. "When the cilia of all 

 these spiral fissures are in active movement at once, the effect is 

 most singular. — Another most remarkable phenomenon presented 

 throughout the group, and well seen in the solitary Ascidian just 

 referred-to, is the alternation in the direction of the Circulation. 

 The Heart, which lies at the bottom of the branchial sac, is com- 

 posed of two chambers imperfectly divided from each other ; one 

 of these is connected with the principal trunk leading to the Body, 

 and the other with that leading to the Branchial sac. At one time 

 it will be seen that the blood flows frorn the Respiratory apparatus 

 to the cavity of the Heart in which its trunk terminates, which then 

 contracts so as to drive it into the other cavity, which in its 

 turn contracts and propels it through the Systemic trunk to the 

 body at large ; but after this course has been maintained for a time, 

 the heart ceases to pulsate for a moment or two, and the course is 

 reversed, the blood flowing into the Heart from the body generally, 

 and being propelled to the Branchial sac. After this reversed 

 course has continued for some time, another pause occurs, and the 

 first course is resumed. The length of time intervening between 

 the changes does not seem by any means constant. It is usually 

 stated at from half-a-minute in the Composite forms to two minutes ; 

 but in the Solitary Ascidian just referred to, the Author has re- 

 peatedly observed an interval of from five to fifteen minutes, and 

 in some instances he has seen the circulation go-on for half-an-hour 

 or even longer without change. 



453. The Compound Ascidians are very commonly found adhe- 

 rent to Sea-weeds, Zoophytes, and Stones between the tide-marks ; 

 and they present objects of great interest to the Microscopist, since 

 the small size and transparence of their bodies, when they are 

 detached from the mass in which they are imbedded, not only 

 enables their structure to be clearly discerned without dissection, 

 but allows many of their living actions to be watched. Of these 

 we have a characteristic example in Amaroucium proliferum ; of 

 which the form of the composite mass and the anatomy of a single 

 individual are displayed in Fig. 302. Its clusters appear almost 

 completely inanimate, exhibiting no very obvious movements when 

 irritated ; but if they be placed when fresh in Sea-water, a slight 

 pouting of the orifices will soon be perceptible, and a constant and 

 energetic series of currents will be found to enter by one set and to 

 be ejected by the other, indicating that all the machinery of active 

 life is going-on within these apathetic bodies. In the tribe of 

 Polyclinians to which this genus belongs, the body is elongated, 

 and may be divided into three regions, the Thorax (a) which is 

 chiefly occupied by the Respiratory sac, the Abdomen (b) which 

 contains the Digestive apparatus, and the Post-abdomen (c) in 

 which the Heart and Generative organs are lodged. At the sum- 

 mit of the Thorax is seen the Oral orifice c, which leads to the 

 Branchial sac e ; this is perforated by an immense number of 



