Tunic at a. 



75 



a circular protrusible basis, supporting the tentacles ; 

 some few are inhabitants of fresh water, and these have 

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

 have a little lobe or epistome overlapping the mouth, 

 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 avicularia, one of the Moss Polypes ; 



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



tentacles, seated on the bases or lophophore around the mouth. 



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 



