BRTOZOA. 



147 



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

 arrangement, 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 (d) ; the other {e, B) 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 and the setose operculum arise near the upper margin of 

 the cell (between e and a, B), and are disposed in six fasciculi, three 

 of which act upon the membrane, and the other tliree upon the 

 bundle of setaj by which it is crowned. When the animal is re- 

 tracted, the setae, which are drawn in after the tentacula, converge 

 and form a kind of defensive operculum. The oesophagus and in- 

 testine are bent into folds : their parietes are contractile. 



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

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



inclosed viscera, and partly 

 by the action of the ali- 

 mentary canal itself. The 

 bundle of setae (./%'. 72., 1., 

 a, b, c) first rises out of the 

 apex of the cell (c), and is 

 followed by the rest of the 

 flexible integument (ib. 2.) : 

 the tentacula next pass up 

 between the setse («?), and 

 separate them ; the folds of 

 the oesophagus and intestine 

 are straightened, and when 

 the act of protrusion is completed, the crown of tentacles expands 

 and their cilia commence vibrating (Jig. 71. c). 



The advantage to Physiology of the researches of the comparative 

 anatomist in the minute forms of animal life, is often very great, in 

 consequence of the favourable conditions which the transparency of 

 the integument, and the distinctness of the contained parts of such 

 animalcules, afford for the direct observation of some of the most 

 recondite and important vital actions. As regards the Bryozoa, the 

 muscles are, as it were, naturally dissected or separated into their 

 component filaments. Each filament generally presents a small knot 



Unfolding of the operculum and upper part of the 

 body, in four stages. Bovoeibankia. 



* XXXV. pp. 394—398. 

 i. 2 



