4 68 



POLYZOA 



l-J 



U-' 



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 

 ap 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 sw r aying 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, 



FIG. ZSS.Bugtdfi turbinata Alder, Plymouth. an( J often strono-lv resemblm*' 

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

 tion of a branch ( x 50) : a, a', avicularia, Seaweeds, Ol 1 tormillg en- 

 indifferent positions ;^, " aperture " (see crus tations on the surface of 

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



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



surrounded by the circle of tentacles; t\vo fpVinrr rm ntlipr <sVm^p Th^ 



individuals to the right show the tentacles takm g on Other Shapes, 



partially expanded ; o, ovicell ; s, marginal units of the Colony are CO111- 



spine. The avicularia of some of the , . -, -, -, ", v - oo<\ 



zooecia have been omitted in B. P lete individuals (1 Ig. 2o4). 



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 (^/O, which is followed by the oesophagus (or), which 

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



