I 4 6 THE SHAPES AND SIZES OF ANIMALS 



egg of an ostrich is perhaps the largest individual 

 cell known to us, though it was very greatly 

 exceeded in recent geological times by that of the 

 gigantic ^Epyornis. It must however be remem- 

 bered that the large size of the yolk of an egg is 

 due mainly to its distension by the granules of 

 food-yolk imbedded in it, which cannot properly be 

 regarded as part of the living substance of the 

 cell. 



In the next group that of the Sponges it is 

 difficult to speak of the size of the individual 

 animals, owing to the very general habit of forming 

 colonies by continuous gemmation, in which 

 colonies the outlines of the component individuals 

 are impossible to determine. The solitary sponges 

 are usually of small size, not exceeding an inch or 

 two in height, but some forms, as the beautiful 

 Euplectella, attain a height of a foot or more. 



Among Ccelenterates the same habit of colony 

 formation prevails, but the boundaries and hence 

 the size of the component members of the colony 

 can almost always be determined. The majority of 

 Ccelenterates are of small size : the individual 

 zooids of a hydroid colony are commonly but a 

 fraction of an inch, rarely much over an inch in 

 length, though occasionally attaining a larger size. 

 The entire colonies often reach great dimensions ; 

 the branching zooids may form brush-like masses 

 some feet across, while the reef-building corals 

 reach a far larger size. 



Speaking generally the Anthozoa, which are the 

 higher group of the two, are of larger size than the 



