SEA SQUIRTS. 



237 



Look at these diagrams : they represent two common forms 

 of Ascidians, and it will be noted that they have a general 

 agreement in shape with the large specimens of A. mentula we 

 were looking at just now. Like that, these have each two 

 necks, though those of mentula were closed, and these are 

 open at their mouths. If we had these in a glass vessel, but 

 still attached to pieces of the rock upon which they grew, we 

 should be able to see why one bottle need have two necks. If 



we were then to drop a little 

 finely-divided colour-powder 

 such as indigo, into the water, 

 we should see two currents 

 were in operation, one flow- 

 ing to the animal, the other 

 proceeding from it. The first 

 would be flowing to the neck 

 marked a in the figures, and 

 the second would be issuing 

 from the mouth of b. Natur- 

 ally, we should at once sup- 

 pose that by means of some 

 internal mechanism and sys- 

 tem of valves, the same 

 current that was being in- 

 duced at a, was being continued through the creature's body, 

 and pumped out at b. Our supposition would be proved 

 correct by the fact that the colour grains streaming in were 

 also streaming out. But what happens to them between 

 entering and departing we cannot clearly see. 



By the aid of another diagram (next page) we may get a better 

 notion of the Ascidian's internal arrangements than by gazing 

 through its integuments. Here are all its parts marked with 

 a letter as a guide to its anatomy. It is a matter of astonish- 

 ment to many fairly intelligent people, to find that such soft 

 creatures as Sea-squirts, Jelly-fishes, Slugs and Caterpillars, 



A. ASCIDIA VIRGINEA. 

 B. CYNTHIA QUADRANGULARIS. 



