POLYZOAN COLONIES. 



249 



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 p . g 



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 Magnified portion of 



Fiustra. 



from the rest. In this respect, 

 therefore, they are utterly unlike the Sertularians, 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, 



