556 



POLYZOA. 



two watchglass-shaped, hard chitinous shells, the edges of which are- 

 usually enclosed by a flat ring formed of cells containing air (float), 

 and sometimes provided with a crown of projecting spines (Fig. 444). 

 The Phyladolaemata generally die down in the winter, and the 

 statoblasts, which germinate in spring and give rise to new colonies, 

 serve the purpose of perpetuating the species. In the fresh-water 

 Ctenostomes, Paludicella and Vidorella, the colonies also die down in 

 winter, and the species is continued by means of certain external 

 buds, which are arrested in development and last through the winter : 

 they are called hibernacula. In some cases (Cristatella, Loplwpus) 

 parts separated off from the colony are able to develop into new 

 colonies, and in Loplwpus the colonies divide spontaneously. 



The Polyzoa are for the most part marine organisms. They are 

 found between tide-marks, and on the floor of the ocean to a depth 

 of 3000 fathoms. Cheiloston/ata only are recorded from depths of 



PIG. 445. a, Larva of ScrnpoceUaria reptans. 6, Larva of Schizoporella (after Barrois). C6 



ciliated disc. 



over 3000 fathoms; Cydostomata are found in depths of 1400' 

 fathoms and less. Ctenostomes are mostly found in depths of less 

 than 40 fathoms. 



One order the Phyladolaemata is exclusively fresh-water, and a 

 few fresh- water forms are known in the Gymnolaemata. A few forms 

 are phosphorescent. Some forms (Terebripora) excavate passages in 

 the shells of Mollusca ; and Hypophorella is found in passages in the 

 tubes of Lanice and Chaetopterus. 



The eggs of the Edoproda are sometimes laid and develop freely 

 in the sea (Hypopliorella e,rpansa, Alcyonidium albidum, Membrani- 

 pora pilosa, Farrella repens). But more often the early stages of 

 development take place under the protection of the parent ; either in 

 the tentacle-sheath (some Ctenostomata}, or, in many Cheilostomata, 

 in a pouch of the zooecium ; or in special zooecia (ooecia), of which 

 the polypide is rudimentary or degenerate (Cydostomata); or in a 



