430 CIRCULATION IN COMPOUND ASCIDIANS. 



tubes, however, does not take place constantly in the same 

 direction. At some periods the heart may be observed to con- 

 tract from behind forwards, so as to propel the blood in the 

 direction just mentioned. After a short time the pulsations 

 become fainter for a few beats, and the flow slower ; and sud- 

 denly, but with a slight pause, the whole current in all its 

 windings is reversed. The heart gives the opposite impulse ; 

 the channel in the peduncle that before poured in the blood now 

 carries it back ; and the other, the contrary. These changes 

 succeed each other alternately, the average time being the same 

 in both directions; but the period of each varying as much as from 

 thirty seconds to two minutes. By this circulation, all the 

 individuals in one group seem to be connected. As in the com- 

 pound Polypifera, increase takes place by sprouts or buds (of 

 which a small one is seen at the right hand of Fig. 602) ; and 

 the two streams of the stem run through the bud, before its organs 

 are developed. The circulation in each individual appears, how- 

 ever, to be independent of the rest ; for it continues when the 

 current through the stalk is interrupted. The stream which 

 returns from the branchial sac and the viscera, is then poured 

 into the posterior part of the heart, instead of entering the 

 peduncle. The circulating system then resembles that of the 

 isolated Ascidise ; and it is remarkable that the regular alterna- 

 tion of the flow may be observed in these also, in every instance 

 in which the integument is thin enough to allow the current of 

 blood to be distinguished. 



980. A closer apparent union between the individuals may be 

 observed in the aggregated Ascidice ; though it may be doubted 

 whether the real connection is as intimate as in the species just 

 described. In these we find a large and variable number of 

 individuals, arranged with more or less regularity within a com- 

 mon envelope. Like the true Ascidiaa, they are all fixed ; some- 

 times forming a slimy crust upon Algae, or other submarine 

 bodies ; sometimes projecting in conical or globular masses ; and 

 not unfrequently spreading over the larger isolated species of 

 Tunicata. They agree with them, also, in the relative positions 

 of the branchial orifice and the funnel, and in the structure of the 



