468 POLYZOA. 



(1211.) The mode in which the protrusion of the tentacula is effected 

 is not so easily explained ; it would seem that the lining membrane of 

 the shell is furnished with circular muscular fibres, so disposed as by 

 their action to compress the fluid contained in the visceral cavity, and 

 thus tend to elongate the body. Dr. Farre, however, believes the ali- 

 mentary canal itself to be the great agent in effecting this object ; and 

 he conceives it to possess a power of straightening itself from the 

 flexures into which it is thrown during the retracted state of the 

 animal. 



(1212.) The ELUSTBJE and ESCHARS are intimately allied to Bower- 

 bankia in all the details of their structure, as we are assured by the re- 

 searches of Dr. Milne-Edwards concerning these singularly aggregated 

 forms of marine Polyzoa*. 



(1213.) The cells of the Flustrce and Escharce are disposed side by 

 side upon the same plane, so as to form a common skeleton of a cori- 

 aceous or horny texture. The individual cells, which are extremely 

 minute, vary in shape in different species ; and the orifice of each is 

 generally defended by projecting spines, or sometimes by a moveable 

 operculum, or lid, that closes the orifice in the contracted state of the 

 animal. The extension of one of these skeletons is effected by the 

 regular addition of new cells around the circumference of the Flustra, 

 those of the margin being, of course, the most recent ; and the latter are 

 not unfrequently found inhabited by healthy animals, whilst in the older 

 or central ones' the original occupants have perished. 



(1214.) The facts observed by Milne-Edwards relative to the forma- 

 tion of these cells possess a high degree of interest, and materially 

 support the views already given concerning the formation of the tubes 

 of zoophytes in general, proving that the calcareous matter to which 

 their hardness is owing is not a mere exudation from the surface of the 

 animal, but is deposited in an organized tegumentary membrane, whence 

 it can be removed with facility by means of extremely dilute muriatic 

 acid. When so treated, a brisk effervescence is produced ; the cells 

 become flexible, and are easily separated from each other ; but they are 

 not altered in form, and evidently consist of a soft and thick membrane, 

 forming a sac containing the digestive organs of the creature. In this 

 state the opening of the cell is no longer denned as it was before, but 

 the membranous cell appears continuous with the tentacular sheath. 

 "We see, therefore, that in these creatures the cell is an integral part 

 of the animal itself not a mere calcareous crust moulded upon the 

 surface of the body being a portion of the tegumentary membrane, 

 which, by the molecular deposit of earthy matter in its tissue, ossifies, 

 like the cartilage of higher animals, without ceasing to be the seat of 

 nutritive movement. It is evident, likewise, that what is called the 



* "Eecherches Anatomiques, Physiologiques, et Zoologiques sur les Eschares" 

 (Ann. des Sci. Nat. for 1836). 



