13 



If an Ascidiaii expanded in sea-water, in a healthy 

 condition, be closely watched, it will soon be noticed that 

 there is a constant stream of water pouring in through 

 the branchial aperture, and another flowing out from the 

 atrial (as represented in fig. 1, PI. II.) ; and if some fine 

 and insoluble coloured powder be dropped into the water 

 near the branchial aperture, it will rapidly be drawn in 

 with the current, and after a short time some of the 

 powder will make its appearance in the water ejected 

 from the atrial aperture. 



Hence, it is obvious that there is a current of sea-water 

 flowing through the body of the Ascidian. This current 

 has four distinct uses or functions : (1) it enables the animal 

 to breathe, by bringing in fresh supplies of oxygen ; (2) in 

 the first part of its course it carries the microscopic food 

 particles into the Ascidian's body ; (3) in the last part of 

 its course it carries out of the body various waste materials, 

 which must be got rid of; and (4) it ejects the mature ova 

 and spermatozoa from the body. The course which this 

 water-current takes is : — in through the branchial aperture 

 to the branchial sac, then through the clefts in the wall of 

 that organ into the surrounding atrium, and lastly out 

 through the atrial aperture to the exterior. The direction 

 of this current may occasionally be temporarily reversed ; 

 and, when the muscular body-wall contracts, a sudden 

 current may be ejected through both apertures simul- 

 taneously. 



All the clefts or stigmata in the wall of the branchial 

 sac (shown in fig. 3, PI. II.) are bounded by cells which 

 bear a number of cilia (PI. IV., fig. 3) projecting across the 

 cleft. These cilia, so long as the animal is alive, are in 

 constant motion, lashing rapidly from the branchial sac 

 towards the atrium, so as to drive the water in the cleft 

 outwards ; and it is this constant action of these very 



