232 AN INTRODUCTION TO ZOOLOGY. 



Some of the marine forms are polymorphic, that is 

 to say, some of the polyps are modified to subserve 

 special uses to the colony. These polyps are degene- 

 rate forms : they are called yibracula and avicularia ; 

 the former bear a long flagellum, hence their name ; 

 the latter are modified into a bird's-head shape very 

 much like that of the pedicellariae of the sea-urchins. 

 The Polyzoa ai-e very numerously represented on our 

 coasts, coating stones or seaweed, or growing free, 

 as in the case of the most common form, Flustra, the 



Pig. 81.— Shell of Terebratula, showing beak of the shell perforated by a 

 foramen for the passage of the peduncle. 



Sea-mat. There are also several fresh-water forms. 

 They all form interesting objects of microscopic study. 

 The Lamp-shells, class Beachiopoda (arm-footed 

 animals), used to be classed with the Mollusca. They 

 have an inequivalve bivalve shell, the two halves of 

 which are placed dorsal and ventral to the animal, 

 and are attached either directly by the shell or by a 

 ligament called the peduncle, which sometimes per- 

 forates the beak, and sometimes passes out between 

 the valves. The feature of the group is the pair of 



