148 THE SHAPES AND SIZES OF ANIMALS 



length attained by some Tapeworms, thirty feet or 

 more, is due rather to their peculiar mode of a 

 sexual reproduction than to increase in individual 

 dimensions. 



Nemertines may attain an enormous length, 

 fifteen or twenty feet being not uncommonly 

 exceeded. Amongst Annelids the average length 

 is certainly below six inches ; but individual genera, 

 as Halla, often exceed three feet, and the giant 

 earthworm of Australia measures six feet or more. 



Rotifers are a very interesting group ; invariably 

 of small size, and often actually microscopic, they 

 yet exhibit very great complexity of organisation. 

 A Rotifer may be actually smaller than a Stentor, 

 and yet while the latter is but a single cell exhibit- 

 ing but very slight differentiation of parts, the 

 former is an animal with well developed cutaneous 

 and muscular systems, a large body cavity, an ali- 

 mentary canal in which the various regions are 

 modified in special manner for special purposes, a 

 definite system of excretory organs, a very perfect 

 nervous system, with well developed and sometimes 

 highly specialised sense organs ; and all these 

 various parts formed of specially differentiated cells. 

 Comparison of a Rotifer with a Protozoon shows 

 us very forcibly that although it may be true that 

 larger animals are, on the whole, more highly 

 organised than smaller ones, yet it is not magnitude 

 alone that determines structure. 



Arthropoda, though an extraordinarily numerous 

 group, far exceeding in number of species all the 

 other groups put together, yet do not present any 



