CHAPTER XIII. 



Polyzoa Annelida Platyhelminthes. 



Polyzoa. 



THIS is a small group of animals which, with one exception, form colonies 

 and live a kind of communal life. Imagine a solitary individual settling down 

 and building a house, and then producing other individuals which occupy 

 separate rooms in this ever-increasing block of tenements, so that at last 

 there is an enormous colony living in the one house, and you have a picture 

 of the complex life of the Polyzoa. The separate members of the colony are 

 exceedingly minute, and must be examined under the microscope. 



Some of them are found in fresh water for example, Cristatella, Lophopus, 

 and Plumatella (illustrated in The Young Peoples Microscope Book] ; but 

 the more familiar forms are marine, and are known as Sea Mats, Sea Moss, 

 Bird's-head Coralline, etc. 



The Sea Mat (Flustra foliacea) is perhaps the most usually observed, for, 

 though it is a deep-sea form, it is thrown up on the beach in large quantities, 

 looking like seaweed hemp-coloured, papery in texture, and built up in 

 fronds much like Fucus senatus. Examined with a strong lens, the fronds are 

 seen to be covered with a network of chambers or cells, in each of which one 

 of the colony lives ; and- the chamber is lined with a horny substance secreted 

 by its inmate, the Polyplie, showing its connection with the Molluscs in their 

 shell-building characteristics. Through a small opening the polypite thrusts 

 its head and fringe of tentacles, thus obtaining food and air. 



Each chamber of the Sea Mat is thus occupied by a complete individual 

 living an independent life, in this way differing entirely from the Zoophytes 

 (among the Hydrozoa), which have a common living base, known as the ccenosarc 

 (" common flesh "). In the case of the Polyzoa the separate individuals live 

 in a common house. Each can multiply by budding, and eggs also are laid, 

 from which a free-swimming larva is hatched, which in tune settles down on a 

 rock and, developing its horny chamber, becomes the founder of another colony. 



One of the most curious facts of such colonies is that not all the individual 

 members are alike some have heads like birds' beaks with snapping bills; 

 others end in a whip-like lash. It appears that these must act in some way for 

 the benefit of the community perhaps as scavengers, soldiers, or police ! 



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