POLYZOAN COLONIES. 



249 



Fig. 182. 



microscopic smallness, which live as neighbours in 

 the horny frond they have secreted in common A 

 magnifying glass will show that the surface of such 

 frond is covered with cells, all shaped alike. In 

 these the polyzoans live, obtaining 

 fresh air and food by means of their 

 cilia, which are protruded so as to 

 be constantly agitating the water. 

 These sea-mats may be kept alive 

 in small aquaria, when even by the 

 naked eye we can perceive the ex- 

 trusion of the cilia by the filmy 

 bluish-whiteness which seems to 

 come over the surface of the frond. 

 Each individual of the colony form- 

 ing the sea-mat lives separately 

 from the rest. In this respect, 

 therefore, they are utterly unlike the Serttilarians, or 

 "sea-firs," in which the individual polpes are con- 

 nected by a common flesh which runs up the horny 

 stem, and is given off to every branch so as to be 

 connected with every zoophyte. Moreover, the ani- 

 mals forming the sea-mats have a nervous system, 

 which the sea-firs have not, as well as a more complex 

 physiological organisation. Some of the members 

 of this family may be seen encrusting sea-weeds 

 with a most delicate white lace-like tracery the 

 lace-work being produced by the cells of the animals. 

 The commonest of these is Membranipora pilosa, 



Magnified portion of 

 Fluslra. 



