POLYZOA 609 



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. 

 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 func- 



ten.8. 



cn.f.s. C;8. ten.8. 



jopc. 



Fig. 417. Protrusion of the polypide in two types of cheilostomatous Polyzoa. 

 Membranipora. 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; p.m. parietal muscles; st. stomach; ten.s. tentacular sheath. 



tional polypide. Further evolution led to displacement of the avicu- 

 laria so that they became appendages of other zooecia, situated near 

 the orifice. The two kinds of modified individuals thus perform tasks 

 which in the Hydrozoa are allotted to the dactylozooids and in the 

 Echinodermata to the pedicellariae. 



Most of the Polyzoa are marine an~d are amongst the most familiar 

 objects of the beach. A complete division, the Phylactolaemata, are 

 freshwater. The marine forms possess a variety of free-swimming 

 larvae, which are of the trochosphere type. In the Phylactolaemata, 

 certain internal buds called statoblasts are formed from lens-shaped 



