464 



POLYZOA. 



of the anatomical details of these creatures in a manner which leaves 

 few points of their economy unknown. 



Fig. 238. 



(1196.) We shall se- 

 lect an individual, named 

 by Dr. Farre Bower- 

 banJcia densa, as an il- 

 lustration of the general 

 structure of the POLYZOA, 

 partly from the complete 

 manner in which its or- 

 ganization has been de- 

 veloped in the memoir 

 alluded to, and partly 

 because we have had 

 frequent opportunities of 

 verifying the accuracy 

 of the observations re- 

 corded. 



(1197.) The tentacula 

 of SowerbanJcia (fig. 238) 

 during the expanded state 

 of the animal are kept 

 quite straight and mo- 

 tionless, as represented 

 in the drawing. Each 

 tentacle is provided upon 

 its outer aspect with a 

 series of stiff and im- 

 moveable spines, proba- 

 bly serving to keep off 

 any foreign bodies that by their proximity might interfere with the 

 ciliary movements immediately to be described. 



(1198.) Besides the stiff spines, the tentacula are covered with an 

 immense number of vibrating cilia, which, at the will of the animal, 

 are thrown into most rapid movement, so as to produce strong and con- 

 tinuous currents in the surrounding fluid, whereby particles floating in 

 the neighbourhood are hurried along with great velocity. Prom the 

 direction of the streams produced by the cilia, namely towards the 

 mouth, we at once perceive the utility and beauty of the contrivance, 

 compensating to a great extent for the fixed condition of the Polyzoon : 

 animalcules floating in the vicinity no sooner come within the influence 

 of the currents so produced than they are forced towards the mouth, 

 situated in the centre of the tentacular zone, and, being at once seized, 

 are immediately swallowed. 



(1199.) The tentacula themselves, notwithstanding their immobility 



Anatomy of Bowerbankia densa (after Farre). a, The 

 animal with its tentacula expanded : 1, pharynx ; 

 2, oesophagus; 3, the gizzard; 4, the stomach; 5, the 

 pylorus ; 6, the intestine ; 7, the anal aperture, b re- 

 presents the Bryozoon retracted into its cell : 1, 2, 3, 

 muscular fasciculi, c, An imperfect gemma before the 

 opening of the cell: 1, stomachal cavity, d, A gemma 

 sprouting from the common stem. 



