99 



BKIT1SH TUNTCATA. 



The chief function of the test, like that of the shell in the 

 higher mollusks, is no doubt to protect the comparatively soft 

 and delicate portions of the animal that lie within it. But it 

 will also act, by its resiliency, as a counterpoise to the muscular 

 contractility of the mantle, which lines it as it were. In those 

 species, such as Styela tulerosa, in which the mantle and test 

 are adherent throughout, this action is readily understood ; it 

 is not,, however, quite so obvious in the species which have 

 these two tunics comparatively free, as they are universally in 

 Ascidia and Molgula. But we have just seen that, in such 



Distal or anterior. 



br.f. Ir.m. 



aa.t. 



Ventral. 



Dorsal. 





Proximal or posterior. 



FIG. 1. Ascidia scabra. Left side. t. Test (outer tunic). m. Mantle 

 (inner tunic), br.t. Branchial aperture of test. Ir.m. Branchial aper- 

 ture of mantle, aa.t. Atrial aperture of test. aa.m. At rial aperture of 

 mantle. Three times natural size. From a specimen preserved in 

 spirit, with the mantle shrunk. 



instances, the inner surface of the test, and the outer surface 

 of the mantle, lie in close contact with each other. Now, as 

 under all ordinary circumstances the pressure of the water 

 inside the mantle must be as great as that of the water rest- 

 ing against the outer surface of the test, and as no water can 

 possibly enter between these two tunics, it is clear enough 

 that they will be held together with no inconsiderable force. 

 Thus, when the muscles of the mantle contract, diminishing 

 the bulk of that organ, the test will be drawn in after it ; and 

 so soon as the muscles of the former relax, the latter, through 

 the elasticity of its walls, will expand, and the mantle will be 

 constrained to do so likewise. 



