ASCIDIANS. 



189 



The Ascidians (Ascidid) * (Fig. 1 88} are met with everywhere in abundance 

 on the shores of the ocean, but very generally are passed unnoticed by the casual 

 observer. In their natural condition they are found fixed to the surfaces of 

 rocks, sea-weed, or other submarine bodies, and frequently glued together in 

 bunches. Incapable of locomotion, and deprived of any external organs of 

 sense, few animals seem more helpless and apathetic than these apparently 



FIG. 183. DIAGRAM OF STRUCTURE OF ASCIDIAX, 



shapeless beings ; and the anatomist is surprised to find how remarkably the 

 beauty and delicacy of their internal structure contrast with their rude external 

 appearance. When we consider the immoveable 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 cal- 

 culated 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 ad- 

 mitted. 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 immedi- 

 ately 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 points that require notice. 

 The Salpians (Salpce] are some of them so transparent that their presence 

 in even a small quantity of sea- water is not easily detected. Their body is 



* dcrKos, askos, a leather bag. 



