146 



LECTURE Tin. 



Each polype presents an oblong depressed, or elongated and 



cylindrical figure, and is protected by 

 a dense integument in the form of a 

 cell or case i^fiQ' 71, a, a), to the mouth 

 of which is attached a sac (^, h') com- 

 posed of very delicate and flexible 

 membrane. This constitutes the upper 

 or anterior integument of the polype 

 when it is protruded, and is reflected, 

 like the inverted finger of a glove, into 

 the firmer portion of the cell when the 

 polype is retracted, as at B. In general 

 the integument forming the firm cell is 

 of a horny texture ; but in the Escharce 

 it is hardened by the deposition of par- 

 ticles of carbonate of lime in the orga- 

 nised animal basis ; so that the external 

 skeletons of the Bryozoa offer analogous 

 conditions to the cartilaginous and bony 

 states of the internal skeletons of fishes. 

 The mouths of the cells in Eschara and 

 Cellepora are provided with a moveable 

 lid with a pair of muscles for closing 

 it, when the polype is retracted.* In 

 the Tendra zostericola there is instead 



Polypes ofaBryozoon, magnified, x 80. of a SOlid OpcrCulum a large flat flcshy 



I'ing, fixed to the walls of the cell by four muscular fasciculi : four 

 other bundles attach the opercular ring to the border of the opening 

 of the cell: the movements of this muscular mechanism are both 

 curious and beautiful when the little polype protrudes from or draws 

 back into its ceU. 



In the cylindrical Bryozoa, as the Boiverhankia, the flexible part 

 of the integument consists of two portions ; the lower half being a 

 simple continuation of the cell ; the upper one consisting of a cylin- 

 drical series of setce {h'), connected together by an extremely delicate 

 and elastic membrane, permitting a certain extension of the cylinder, 

 which, at the same time, supports and allows free motion to the upper 

 part of the body in its expanded state. The mouth of the polype is 

 situated at this extremity of the body, and is surrounded by a 

 radiated series of slender, ciliated tentacula (c), eight, ten, twelve, or 

 more in number, according to the genus. 



The muscular system is developed in the present highly organised 



* CXXVII. p. 25. pi. i. fig. 1. e and 



