442 POLYZOA ; FLUSTKA. 



of the protrusion of the animal. This is the case in the common 

 Flustra of our own coasts, which presents a flat expanded sur- 

 face, so thickly set with these apertures, as to appear like a deli- 

 cate network. These Sea-mats, as they have been fancifully 

 termed, so much resemble common sea-weeds in their general as- 

 pect, that they are often mistaken for them by ignorant collectors. 

 They may readily be distinguished, however, by the crispness 

 of their feel, when contrasted with the flabbiness of the Algse, 

 as well as by the polype-cells so beautifully arranged on their 

 surface. The extension of these compound structures takes 

 place by buds which are developed from the outer surface of the 

 cells. From one original cell of the Flustra, five such buds may 

 be sent off, which develope themselves into new cells around it ; 

 and these, in their turn, produce buds from their unattached 

 margins, so as rapidly to increase the number of cells to a very 

 large amount. This extension may go on almost without limit ; 

 and it often happens that the cells in the centre of a leaf-like 

 expansion of the Flustra are devoid of inhabitants, and have 

 lost their vitality, whilst the edges are in a state of active growth. 

 The establishment of new colonies is provided for by the pro- 

 duction of ova at certain periods. These are formed in organs 

 attached either to the intestine or to the inner wall. They escape 

 from the cavity of the body by a small aperture in the vicinity 

 of the anus, and give birth to a ciliated embryo, which swims 

 about for some time, but finally fixes itself, forms a cell, and be- 

 comes a simple Polyzoon, from which a new compound structure 

 is produced by gemmation in the manner above described. 



1077. The animals forming these compound structures are 

 usually packed closely together ; and are of very minute size ; 

 so that a single polyzoary of very moderate dimensions must 

 contain an enormous multitude of individuals. Dr. Grant has 

 reckoned that, on an ordinary specimen of Flustra carbasea, 

 there are about ten square inches of surface ; in each square inch 

 there may be about 1800 cells, thus making all together 18,000 

 within this small space. Each of these animals has 22 tenta- 

 cula ; so that there will be about 396,000 of these minute arms 

 upon this little specimen. If each of these tentacula has only 



