POLYZOA 533 



funiculus and are enclosed by chitinous shells. The polypidesdie down 

 during the winter and in the spring the statoblasts germinate and 

 produce new colonies. 



Polymorphism is a feature of polyzoan as it is of hydrozoan colonies. 

 Perhaps the most remarkable modifications are to be seen in the in- 

 dividuals known as vibracula and avicularia of such forms as Bugula 

 (Figs. 393 A, 394). The vibracula are nothing more than long bristles 

 which are capable of movement and often act in concert throughout a 

 part of the colony, sweeping backwards and forwards over the surface, 

 preventing larvae and noxious material from settling on the colony. 



ten.s. 



ca.f.s. 



c.s. ten.s. 



^.opc. 



p.m.- 



Fig. 392. Protrusion of the polypide in two types of cheilostomatous Polyzoa. 

 Memhranipora. After Harmer. A, With polypide retracted. B, With polypide 

 protruded. C, A form with a calcareous frontal wall. An. anus; ca. calcified 

 cuticle of zooecium ; c.s. compensation sac ; f.s. frontal surface ; ga. ganglion ; 

 int. intestine; oe. oesophagus; ope. operculum; m.retr. retractor muscle of 

 polypide; ^.m. parietal muscles; st. stomach; ten.s. tentacular sheath. 



The avicularia resemble the head of a bird, possessing a movable 

 mandible which is homologous with the operculum of an unmodified 

 polyp, and this is provided with powerful muscles. The avicularia 

 suddenly snap their jaws and enclose as in a vice small roving animals 

 which touch them, particularly the larvae of incrusting animals. In 

 the most primitive cases, an avicularium is found in the same position 

 in the colony as an ordinary zooecium and may even possess a 

 functional polypide. Further evolution led to displacement of 

 the avicularia so that they became appendages of other zooecia, 

 situated near the orifice. The two kinds of individuals thus perform 



