54 A HALF-HOUR WITH THE 



In each one of these is seated a creature of much 

 more complicated organization than the polyps 

 just examined. It has, it is true, a ring of ten- 

 tacles ; but if these are examined, the tentacles 

 are found to be covered with cilia (Fig. 159, a, 

 PI. 6). This family of creatures are called Polyzoa, 

 and form a group of animals which are classed 

 with the Mollusca, or shell-fish. Sometimes these 

 creatures attach themselves to sea-weeds, oysters, 

 stones, and other objects at the bottom of the sea, 

 forming a kind of cellular membranous expansion. 

 Such are the species of Lepralia (Fig. 153, PL 6). 

 Sometimes the cells are elongated and elevated 

 above the surface of the object on which they are 

 placed, as in the case ot B&werbankia, (Fig. 154, 

 PI. 6). A beautiful form of these creatures is the 

 shepherd's-purse coral (Fig. 155, PL 6). This 

 creature belongs to a group of the polyzoa, remark- 

 able for possessing little processes on the margins 

 of their cells, in shape resembling the bowl of a 

 tobacco-pipe. On examining them with the Micro- 

 scope, they present a very complicated organization : 

 they possess two jaw-like processes, which open and 

 shut like a bird's bill, and from this fact they have 

 been called avicularia, or bird's-head processes (a). 

 In other species, as in Bugula avicularia (Fig. 156, 

 PL 6), these creatures possess not only the bird's- 

 head process, but a second, consisting of a long 

 bristle or seta, attached by a joint to a process 

 below (&). These bodies are called vibracula, and 

 the bristle-like extremity is kept constantly in 

 action. Few objects are more curious under the 

 Microscope than these avicularia and vibracula in 

 a state of action. Whilst the function of the 

 vibracula (a) seems to be to sweep away objects 

 that would interfere with the life of the animal 

 in the cell, it has been suggested that the avi- 



