532 CILIOBRACHIATE POLYPES; BOWERBANKIA ; FLUSTRA. 



respiration. That a part of the alimentary canal should be 

 specially modified for this purpose, is by no means surprising ; 

 for such a modification, in a somev.hat different form, is not at 

 all uncommon in various divisions of the animal kingdom. 

 Thus, in the lowest group of Mollusca, to which these Polypes 

 bear considerable affinity, we have seen that the entrance to the 

 stomach lies at the bottom of a large cavity lined by the respira- 

 tory membrane, over the walls of which currents of water are 

 constantly passing, which supply the digestive organs with 

 alimentary materials, besides effecting the aeration of the blood 



(8 974). 



1 104. As in the two preceding Orders, we meet with species 

 among the Ciliobrachiate polypes, which have the power of 

 forming stony cells, by a deposit of 'lime in their soft tissues. 

 The cells of some of these polypes are covered-in, when the 

 animal is withdrawn, by a sort of lid or flap, which is provided 

 with muscles for drawing it down upon the mouth of the cell. 

 In others of the compound polypidoms, however, the cells over- 

 lap one another obliquely ; and the orifice is not at their extre- 

 mity but rather on one side, so that the operculum has only to 

 be slightly upraised, to allow of the protrusion of the animal. 

 This is the case in the common Flustra of our own coasts, which 

 presents a flat expanded surface, so thickly set with these 

 apertures, as to appear like a delicate network. These Sea-mats, 

 as they have been fancifully termed, so much resemble common 

 seaweeds in their general aspect, that they are often mistaken 

 for them by ignorant collectors. They may readily be distin- 

 guished, however, by the crispness of their feel, when contrasted 

 with the flabbiness of the Algae, 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 



