426 



GENERAL STRUCTURE OF TUNICATA. 



cavity of the mantle, not only the fluid stream which has 

 passed over the walls of the branchial sac, and has served its 

 purpose in aerating the blood, but also the solid particles which 

 are rejected from the alimentary canal, and the ova which are 

 discharged, when mature, from the ovaria. The heart, which is 

 usually situated close to the digestive organs, is of a more or less 

 tubular form ; and the circulation of the blood is effected by the 

 progressive contraction of the walls of the heart from one end 

 to the other. The circulation, however, differs in a most re- 

 markable manner from that of all other animals. Instead of 

 circulating constantly in one direction, the blood is driven out of 

 the heart continuously towards one part of the body for a certain 

 time ; the contractions of the heart then cease, and after a short 

 repose, recommence in a direction opposite to the former one. 



1057. Between the two orifices there is 

 a nervous ganglion, which sends filaments 

 to each of them, and distributes its principal 

 branches over the general surface of the 

 mantle. No organs of special sensation, 

 however, are perceptible ; except the rudi- 

 ments of an auditory organ in some species ; 

 and the only indication of common sensi- 

 bility shown by these animals, is the con- 

 traction of the mantle when they are touched, 

 by which the water contained in the bran- 

 c hj a i sac i s spirted out, sometimes to a con- 



7 



siderable distance. Sometimes a number of 

 them are so closely impacted together on the 

 rocks > that the impression given to one 

 causes it suddenly to retract, which acts also 

 on the one next to it, and so on throughout several of them ; 

 and each in contracting throws out a quantity of water. After 

 the contracting force has ceased to operate, the usual form is re- 

 stored by the elasticity of the tunic. 



1058. In many cases, however, no regular movements of 

 this kind are commonly employed, either for the respiratory pro- 

 cess, or for the prehension of food. A continuous and equable 



FIG. 701. -NERVOUS SYS- 



TEM OF ASCIDIA : , 



branchial orifice or 

 uonTi mlntie (t'heT- 

 move!) C at being "" 



' 



