468 



POLYZOA 



that this structure moves backwards and forwards in a deliberate 

 but determined fashion, its lower jaw usually widely open so as 



to be nearly 180 distant 



from its position when closed. 

 Suppose that the lower jaw 

 is moved by powerful muscles 

 which can be distinctly seen 

 inside the transparent head 

 of the avicularium, and that 

 every now and then it closes 

 with a snap, seizing any un- 

 fortunate worm which may 

 happen to be within reach 

 with a grasp of iron. The 

 above gives a very faint idea 

 of the appearance of a living 

 Bugula colony, with its hun- 

 dreds of swaying avicularia, 

 and with its tentacular funnels 

 protruding from their zooecia, 

 and withdrawing themselves 

 capriciously from time to 

 time. 



General Characters. 

 The Polyzoa are colonies, 

 leaf-like or tree-like in form, 



avicularia, seaweeds, or forming en- 

 aperture " (see crus tations on the surface of 



Fig. 233. Bugula turbinata Alder, Plymouth. an( J ft en strongly resembling 

 A, A small colony (natural size) ; B, por- 

 tion of a branch ( x 50) : a, a' 

 in different positions ; op, 

 p. 524) ; b, polypide-bud, attached by its 



stomach to b.b, brown body ; m, mouth, stones and Water-plants 

 surrounded by the circle of tentacles; two taM Qn th fa 

 individuals to the right show the tentacles o r 



partially expanded ; o, ovicell ; s, marginal units of the Colony are COm- 

 spine. The avicularia of some of the 

 zooecia have been omitted in B. 



or 

 The 



plete individuals (Fig. 234). 



The zooecium or body -wall 

 encloses a body-cavity, in which lies a digestive canal, with which 

 are closely connected the central nervous system and the retractile, 

 ciliated tentacles. The structures other than the zooecium con- 

 stitute the " polypide." The mouth (m) leads into the ciliated 

 pharynx (ph), which is followed by the oesophagus (oe), which 

 again passes into the stomach (s), whose walls are coloured by a 



