224 TUNICATA, OR ASCIDIANS. PAKTIII, 



to flow. A short pause takes place, the heart gives an 

 opposite impulse, and the blood makes its circuit in a 

 direction exactly contrary to what it did before. The 

 circulation in all these little globes is brought into con- 

 nection by a simultaneous circulation through two tubes 

 in the silvery thread to which they are attached. 



The average duration of the ebb and flow of the 

 blood is probably the same, but the period between the 

 changes varies from thirty seconds to two minutes. As 

 the blood is colourless and transparent, it probably 

 would have been impossible to determine its motion had 

 it not been for solid particles floating in it. 



The larva of the compound sessile Ascidians is like 

 the tadpole of a frog, which swims about for a time ; it 

 then fixes itself by the head to some object, the tail falls 

 off, and in a few days it becomes a solitary Ascidian, 

 with its two orifices and currents of water. This soli- 

 tary animal gives origin by budding to a connected 

 group, which in its turn lays fertilized eggs, so that 

 there is an alternation of generations. 



The Botryllidse or Star-like Ascidians, appear as masses 

 of highly coloured gelatinous matter, spread over stones 

 or fiici in which from ten to twenty minute oblong As- 

 cidians are arranged in a circle round a common open 

 centre which is the discharging orifice of the whole 

 group, for the mouth of each individual is at the oppo- 

 site extremity. The only indication of life given by this 

 compound creature is the expansion and contraction of 

 an elastic band surrounding the discharging orifice. 

 The organization of each of these individuals is similar 

 to that of the Perophora. 



Although many Tunicata form composite societies, 

 the most numerous and largest in size are always soli- 

 tary, as the Ascidia virginea (fig. 162). Its outer 

 tunic contains cellulose, it is pale and semitransparent, 

 the inner tunic is orange-coloured or crimson. These 



