96 



LECTUKE VIII. 



very delicate and flexible membrane. This constitutes the upper or 

 anterior integument of the polype when it is protruded, and is re- 

 flected, 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 in- 

 tegument forming the firm cell is of a horny texture ; but in the 

 Escharce it is hardened by the deposition of particles of carbonate of 

 lime in the organised 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. 



In the cylindrical Bryozoa, as the Boiverbankia, 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 cylindrical 

 series of setae {b'), 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 

 class of polypes, in the form of distinct groups of fibres. Their ar- 

 rangement, and the actions by which they effect the protrusion and 

 retraction of the polype, are minutely and clearly described by Dr. 

 Farre. The retractor muscles form two series, one acting upon the 

 alimentary canal, and the other upon the flexible part of the cell. 

 One series rises from the bottom of the cell, and is inserted about the 

 base of the stomach (t?) ; the other (e) arises from the opposite side of 

 the bottom of the cell, and passes upwards to be inserted near the base 

 of the tentacula. The muscles which retract the flexible integument, 

 arise near the upper margin of the cell, and are disposed in six 

 fasciculi, three of which act upon the membrane, and the other three 

 upon the bundle of setae by which it is crowned. When the animal 

 is retracted, the setae, which are drawn in after the tentacula, converge 

 and form a kind of defensive operculum. The oesophagus and intestine 

 are bent into folds. 



The protrusion of the animal is effected, partly by the action of 

 short transverse muscular filaments (e), which tend to compress the in- 

 closed viscera, and partly by the action of the alimentary canal itself. 

 The bundle of setae first rises out of the apex of the cell, and is 

 followed by the rest of the flexible integument : the tentacula next 

 pass up between the setae, and separate them ; the folds of the 

 oesophagus and intestine are straightened, and when the act of 



