33° 



ANIMAL DEVELOPMENT 



growth takes place in front of this, a second division follows, and 

 so on. In the temporary chain of individuals thus formed the 

 youngest is therefore the one immediately behind the original 

 animal. In other species a more complex arrangement may be 

 found. 



A more remarkable case is that of a Bristle-Worm (Syl/is 

 ramosa, fig. 856), in which lateral budding takes place, so that 

 a temporary branching colony is brought into existence. This 

 creature lives within the body of a sort of sponge. 



Vegetative Propagation and Regeneration in Moss- Polypes 

 (^Polyzoa). — The vast majority of Moss- 

 Polypes are colonial organisms and, as in 

 Hydroid Zoophytes, the colonies result 

 from a process of budding, and differ 

 largely in shape according to the way in 

 which this takes place. 



Our native freshwater species die down 

 in autumn, but before this takes place zvin- 

 ter-buds are produced (fig. 85 7), which 

 remain dormant till the following sprino-, 

 when they develop into new colonies. 

 This affords another example of the 

 method of tiding o\'er an unfavourable 

 season, which has already been described 

 in the case of the Freshwater Sponge (see p. 326). 



The winter-buds of Moss- Polypes are sometimes produced 

 externally, but more commonly they arise internally, being known 

 in that case as statoblasts. It is usual for these structures to be 

 surrounded by a ring of air- containing cells, and in one case 

 {Cristatella) there is also a circlet of grappling-hooks, which are 

 likely to catch in water-plants or other objects. Harmer (in The 

 Cambridge Natiiral History) makes the following interesting 

 remarks regarding the buoyancy of these winter-buds: — "The 

 production oi floating statoblasts may seem a strange adaptation 

 to the conditions of freshwater life, since it might be assumed, 

 a priori, that these structures would be specially liable to be 

 frozen during the winter. The following e.xperiments made by 

 Braem show, however, that the germinating power of the stato- 

 blasts is improved by a certain amount of frost. A number of 

 statoblasts were taken; half of these were placed in water, which 



g. S56. — Part of Branching Colony 

 of an Annelid {SyUis rin)wsa) 



