go WORMS, ROTIFERS, LEECHES, POLYZOA 



that it is composed of hundreds of separate minute chambers, 

 placed back to back, each rounded at one end, near which is the 

 narrow opening through which the tentacles of the living animal 

 are pushed. Each minute tenant of one of these cells has a sac- 

 like or cup-shaped body within which can be seen by the aid of 

 the microscope the bent U-shaped food canal ; while around the 

 mouth there is a beautiful wreath of tentacles. All the Polyzoa 

 multiply by budding, and in this way the area and the number of 

 individuals making up a colony increase ; but the Polyzoa also 

 reproduce sexually, the sexes being either separate or united. 

 The young, or larvae developed from the egg, are free-swimming, 

 and become the founders of the new colonies. 



The members of a colony are not always all alike, some of them 

 being modified in a very remarkable manner, appearing as organs 

 resembling a bird's head, or as a long whip-like lasher, and termed 

 respectively the avicularium and vibraculum. The curious " bird's 

 heads " are highly developed in the Bird's-head Polyzoa (Bugula 

 avicularia), and present the most extraordinary appearance under 

 the microscope. Beside the ordinary tentacle-crowned polyp 

 members of the colony there are scattered over its surface a large 

 number of these curious " bird's-head " organs, which look just 

 like miniature vulture's heads mounted on slender, contractile 

 necks which sway from side to side, and armed with sharp beaks 

 which open and shut with a vicious and angry snap upon any- 

 thing that comes within their reach. These organs and the slender 

 lashers which replace them in certain species appear to act as 

 cleansers, preventing sediment and many small creatures from 

 settling upon the colony. The fresh-water forms of Polyzoa 

 reproduce by curious winter-buds, called statoblasts, which are 

 liberated on the death of the parent, are floated away, and after 

 remaining dormant throughout the winter months develop into 

 new colonies in the spring. 



Very numerous in the seas of to-day, the Polyzoa flourished 

 in the seas of past geological epochs, their beautifully preserved 

 fossil remains being very plentiful in Silurian and all the more 

 recent formations. Although some colonies of Polyzoa bear a 

 resemblance to the Hydrozoa, it is but superficial, for the indivi- 

 dual animals are much more complex in structure, and are inde- 

 pendent of each other. Cristatella, Lophopus, and Plumatella are 



