TUNICATA. 251 



prised to find how remarkably the beauty and delicacy 

 of their internal structure contrast with their rude 

 external appearance. When we consider the im- 

 moveable condition of an Ascidian, and its absolute 

 want of any prehensile instruments with which to 

 seize prey, it is by no means easy to conjecture how 

 it is able to subsist ; neither is the structure of the 

 mouth itself, nor the strange position that it occupies, 

 at all calculated to explain this part of their economy. 

 Their mouth is, in fact, situated at the bottom of a 

 wide bag, into which the surrounding water is freely 

 admitted. The internal surface of the bag is densely 

 covered with cilia, which, in the living animal, are 

 constantly in a state of rapid vibration, hurrying along 

 whatever substances, alive or dead, may be brought 

 into the body with the external element, and pouring 

 them into the mouth, when they are immediately 

 swallowed. Many forms of Tunicated Mollusca are 

 met with in the seas of tropical latitudes, which, 

 although allied to the Ascidians in the main points 

 of their economy, differ from them in some particulars 

 that require notice. 



The Salpians (Salpde) are some of them so trans- 

 parent that their presence in even a small quantity 

 of sea water is not easily detected, Their body is 



FlG. 190. SALPA MAXIMA. 



oblong, and open at both ends, the posterior opening 

 being very wide, and furnished with a valve so dis- 

 posed that water is freely admitted, but cannot again 



