BRYOZOA. 



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 

 alimentary 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. 



(148.) The FLUSTILE and ESCHARS are intimately allied to 

 Bowerbankia in all the details of their structure, as we are 

 assured by the researches of Dr. Milne Edwards concerning these 

 singularly aggregated forms of Bryozoa.* 



The cells of the Flustrte and Eschara are disposed side by 

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

 coriaceous or horny texture. The individual cells, which are ex- 

 tremely minute, vary in shape in different species ; and the orifice 

 of each is generally defended by projecting spines, or sometimes 

 by a moveable operculuin, or lid, which 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. 



The facts which have been observed relative to the formation 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, 

 from which it can be removed with facility by means of extremely 

 dilute muriatic acid. When so treated, a brisk effervescence is pro- 

 duced, 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 de- 

 fined as it was before, but the membranous cell appears continuous 

 with the tentacular sheath. We see, therefore, that in these crea- 

 tures the cell is an integrant part of the animal itself, not a mere 



* Recherches Anatomiques, Physiologiques, et Zoologiques sur les Eschares. Ann. 

 des Sciences Nat. for 1836. 



