Tunicata, 



a circular protrusible basis, supporting the tenta- 

 cles ; some few are inhabitants of fresh water, and 

 these have the tentacles on a horse-shoe-shaped basis ; 

 these also have a little valve to shut the mouth, which 

 is present in only two of the marine forms. Each of 

 the little constituent animals of one of these colonies 

 has its own digestive canal, its own nervous system, 



FIG. 45. 



A. Natural size of Acamarchis avicuJaria, one of the Moss Polyps ; 



B. Magnified view of one Polype, showing its 'bird's head.' 



and its own egg-producing apparatus, and these are 

 essentially like the corresponding organs in worms. 



CLASS X. Tunicata. These also are marine soft- 

 bodied animals, met with in abundance attached to 

 shells and stones among the tangles on our sea shores. 

 They are often called sea- squirts, on account of their 

 ejecting little jets of water from their terminal open- 

 ings when irritated. They appear as irregular or oval 



