40 THE ASCIDIA. 



Through the orifice a, both the water required for 

 aerating the circulating fluid, and also the particles of 

 food, are taken in ; through the orifice b the excre- 

 mentitious part of the digested aliment is rejected. Now, 

 if a portion of this tunic be removed, we find it lined 

 with a serous vascular membrane, reflected at each 

 orifice over the body of the animal, which, united only 

 to the tunic at these points, hangs loosely within. 



The body itself is immediately invested with a mus- 

 cular tissue or mantle, the fibres of which cross each 

 other in various directions, and which by its contraction 

 is capable of compressing the internal viscera with great 

 force. Opening the muscular tissue, we find the greater 

 part of the body to be occupied by a large sac, or mem- 

 branous cavity, communicating with the orifice a. This 

 sac is fine, and exquisitely vascular, blood-vessels, ex- 

 ceedingly minute, ramifying and forming a net-work 

 over it ; besides this, its inner surface is covered with 

 minute vibratile filaments, or cilia, in constant agitation. 

 This cavity is the respiratory or aerating apparatus ; it 

 is filled with water through the orifice a, and the action 

 of the cilia efi'ects a current in it, fresh portions being 

 thus perpetually brought into contact with the blood- 

 vessels. By the muscular action of the mantle, the 



