HIGH SCHOOL ZOOLOGY. 



241 



Neither are special circulatory or respiratory organs present, 

 but a pair of excretory tubes put the ccelom in communication 



with the outside. In addition to in- 

 crease by budding, the Fresh-water 

 Polyzoa give rise to new colonies by 

 winter-buds or statoblasts, which, are 

 protected by a double hoi-ny shell, (often 

 ornamented with hooks — Pectinatella, 

 Cristatellu — (Fig. 1G9). These float np 

 to the surface in Spring and give rise 

 at once to new colonies by budding. 

 In the marine forms, the colonies are frequently jpo?y;/4orp7iic, 

 special individuals being modified for prehension alone, others, 

 so as to act as brood-pouches for the developing eggs. Fossil 

 remains of such as had calcareous ccencoecia {e.g, Fenestella) are 

 abundant fx'om the Silurian strata upwards. 



Fig:. 169.^Statoblast of Cnsta- 

 tella. X 25 



