FLATWORMS AND MESOZOA, ETC. 63 



that they possess a superficial resemblance to certain plants, and 

 it must be remembered that this habit of growth is assumed by 

 many animals that have nothing to do with the Polyzoa. The 

 term "Coralline" is sometimes applied to those calcareous 

 Polyzoa which grow into coral-like forms ; and the Tertiary 

 deposit known as " Coralline Crag " is so called from the large 

 number of fossil Polyzoa which it contains. 



"The Polyzoa are none the less a most attractive group. 

 Let anyone examine a dry piece of brown, paper-like substance 

 (Flustra foliacea) which may be found thrown up on the beach 

 on many parts of our coast. Of this species, the so-called " sea- 

 mat," an old writer says : " For curiosity and beauty, I have not, 

 among all the plants and vegetables I have yet observed, seen 

 any one comparable to this sea-weed."* Viewed with a 

 microscope the frond is seen to consist of two layers, placed back 

 to back, of oblong chambers, each of which is the dried body-wall 

 of a single individual. The whole is obviously a colony, and to 

 this fact the term Polyzoa refers 



There is hardly a more surprising spectacle in the whole 

 animal kingdom than a living fragment of the genus Bugula. 

 The colony grows in the shape of a small tree, whose height may 

 amount to several inches ; and is characterised in many species 

 by a spiral arrangement of the branches, which makes the genus 

 easy to recognise at first sight (Fig. i, A). The stem and 

 branches are composed of a single layer of Zooecia, arranged two 

 or more abreast. Each zooecium bears on its outer side a most 

 singular body, termed an avicularium, from its resemblance to a 

 bird's head. Imagine a minute eagle's head attached by a short 

 but flexible neck to the zooecium. Suppose, further, that the 

 structure moves backwards and forwards in a deliberate fashion, 

 its lower jaw usually open so as to be nearly i8o^ distant 

 from its position when closed. Suppose that its 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 unfortunate worm which 

 may happen to be within reach with a grasp of iron. The above 

 gives a faint idea of the appearance of a living Bugula colony, 



*Hooper, quoted by Landsborough, Hist, Brit. Zoophytes, 1852, p. 346. 



