300 DECEMBER. 



gether side by side, of which the one is considerably 

 higher than the other. 1 



We have before us one of the Tunicata, an order of 

 molluscous animals which are closely allied to the Con- 

 chifera or bivalves, but somewhat lower in the scale 

 than they. It has no shell ; that is to say, lime is not 

 deposited in the outer investment, so as to give it the 

 hard, rigid, solid texture of shell; but the internal 

 organs, which are essentially similar to those of an 

 Oyster or a Sand-gaper, are enclosed in a tough leathery 

 coat, known as the test, which is in fact a closed shell 

 destitute of lime. The eminent physiologist, John 

 Hunter, who had dissected some of these homely Squirt- 

 ers, as they are familiarly called, recognised, with his 

 wonted acumen, the structural similarity of their 

 leathery envelopes to the stony shells of the lower 

 bivalves ; and, associating them in a group, called 

 them " soft-shells." The naturalness of this group, since 

 called Tunicata by Lamarck, has been recognised by 

 modern zoologists. 



If we watch our Ascidia for a few minutes, we per- 

 ceive that at irregular intervals one or both of the 

 gaping orifices are suddenly closed and contracted, 



1 Ascidia ? ; perhaps A. prunum ; but much uncertainty rests on 



the names of the Tunicata. The species is represented in Plate xxxiv : 

 two specimens on the right hand. 



