THE SHAPES AND SIZES OF ANIMALS 137 



equally well marked ; the muscular, skeletal, diges- 

 tive, circulatory, nervous, and reproductive systems 

 all extend radially and symmetrically along the 

 arms. There are no anterior or posterior ends, 

 right or left sides to the animal, for in its dilatory 

 ramblings a starfish moves indifferently in any 

 direction, any one of its five arms leading. 



Bearing in mind what has been said above as to 

 the primitive nature of radiate symmetry, and of 

 the relation between it and bilateral symmetry, 

 bearing in mind also that Echinodermata are not 

 merely all aquatic, but are exclusively marine, and 

 that there are the most cogent reasons for regard- 

 ing the marine fauna as the primitive one, from 

 which both the fresh-water and terrestrial have 

 sprung, we should I think naturally conclude that 

 the radiate symmetry of Echinodermata is primitive, 

 and that a starfish is a radiate animal, which has 

 adopted crawling rather than pelagic habits, pre- 

 sumably for convenience in obtaining food, and in 

 which consequently a distinction between ventral 

 and dorsal surfaces has been established ; but that 

 the further structural modification by which anterior 

 and posterior ends, right and left sides, become 

 differentiated, has not yet appeared in it. If we 

 want further evidence in support of this primitive 

 character of Echinodermata, we may obtain it from 

 the past history of the group, for Echinoderms are, 

 geologically considered, a group of extreme antiquity, 

 and a group in which the characteristic radiate sym- 

 metry is as marked in the older as in the more 

 recent members. If however we consider the 



