50 ASCIDIANS 



tioii pass along the heart from end to end, first for a certain 

 number of heats in one direction, and then, after an interval, in 

 the other. If a small or young Ascidia be placed alive, left 

 side uppermost, in a watch-glass or small trough of sea-v^ater, 

 and examined vsdth a low power of the microscope, the heart will 

 be readily seen near the posterior end of the transparent body. 

 It will be noticed that the " beating " looks like successive waves 

 of blood pressed through the tubular heart from one end to the 

 other by its contractions. After watching the waves passing, let 

 us say, from the right hand end of the heart to the left for about 

 a minute and a half (perhaps 60 or 80 to 100 beats), it will 

 be seen that they gradually become slower and then stop 

 altogether. But after seven or eight seconds a faint wave of 

 contraction will start from the left end of the heart and pass 

 over it to the right ; and this will be followed by larger ones 

 for a minute and a half, and then again a pause will occur and 

 the direction change. It has been suggested that the cause of 

 this remarkable reversal may possibly be that the heart being on 

 the ventral vessel, which is wider than the corresponding dorsal 

 trunk, pumps the blood into either the lacunae of the branchial 

 sac or those of the viscera in greater volume than can possibly 

 get out through the smaller branchio- visceral vessel in the 

 same time, the result being that the lacunae in question soon 

 become engorged, and by back pressure cause the stoppage, and 

 then reversal of the beat. The absence of any valves in the 

 heart to regulate the direction of flow obviously facilitates this 

 alternation of the current. 



The larger channels through which the blood flows may be 

 lined with a delicate endothelium, but the smaller passages are 

 merely spaces in the connective tissue. The heart, although 

 anatomically a " ventral vessel," runs in the main dorso-ventrally. 

 The blood-channel leaving the ventral end of the heart is the 

 " branchio-cardiac vessel " (Fig. 23, l.c). This gives off a branch 

 which, along with a corresponding branch from the "cardio-visceral" 

 vessel {c.v) at the other end of the heart, goes to the test, and then 

 runs along the ventral edge of the branchial sac as the branchial 

 aorta {h.a), external to the endostyle, communicating laterally 

 with the ventral ends of all the transverse vessels of the branchial 

 sac. The cardio-visceral vessel (Fig. 23, c.r) after givino- off its 

 branch to the test breaks up into a number of sinuses which 



