GENERAL STRUCTURE OF TUNICATA. 425 



from the water introduced into it for the purpose of respiration. 

 The oesophagus is short, and leads to a capacious stomach, 

 surrounded by clusters of biliary follicles ( ANIM. PHYS. 356), 

 the rudimentary form of a liver. The intestine generally makes 

 one or two turns in the space between the branchial sac and the 

 mantle; and terminates in the neighbourhood of the funnel. 

 The ovaria are usually large, and lie amongst the viscera ; their 

 excretory duct also terminates in the same situation. The fun- 

 nel thus serves to carry out of the 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. 

 973. Between the two orifices there is a nervous ganglion, 

 which sends filaments to each of them, and distributes its prin- 

 cipal branches over the general surface of the 

 mantle. No organs of special sensation, 

 however, are perceptible ; and the only indi- 

 cation of common sensibility shown by these 

 animals, is the contraction of the mantle 

 when they are touched, by which the water 

 contained in the branchial sac is spirted out, 

 sometimes to a considerable distance. Some- 

 times a number of them are so closely im- 

 pacted together on the rocks, that the 

 impression given to one causes it suddenly to 

 Fro. SOL NKBVOUS SYS- re tract, which acts also on the one next to 



TEM OF ASCIDIA ; O, . 



branchial orifice or ' it, and so on throughout several of them ; 

 ntio and each in contracting throws out a quan- 



<the external coat be- tity of water. After the contracting force 



ing removed). , , 



has ceased to operate, the usual form is 

 restored by the elasticity of the tunic. 



974. No regular movements of this kind, however, are com- 

 monly employed, either for the respiratory process, or for the 

 prehension of food. A continuous and equable current of water 

 enters the branchial orifice, and is propelled by the funnel, with- 

 out any other physical agency that can be perceived, than the 



