THE SHAPES AND SIZES OF ANIMALS 147 



Hydrozoa ; and it is also among the more highly 

 organised groups that the largest individuals, the 

 giant Coelenterates, are met with. Thus, of the 

 two groups of jelly-fish, the Hydromedusae, which 

 belong to the Hydrozoa, are small, while the 

 Scyphomedusse, which in many points of structure 

 and also in some peculiarities of development are 

 more nearly allied to the Anthozoa, are of much 

 greater average size and occasionally reach ex- 

 traordinary dimensions, individuals measuring as 

 much as seven feet in diameter with tentacles 

 nearly fifty feet in length having been met with. 

 Sea anemones again, the typical Anthozoa, are 

 much larger than the hydroids ; a height of three 

 or four inches with a diameter of one or two 

 inches is not at all uncommon, while from tropical 

 seas anemones have been described over three feet 

 in diameter. 



The Echinodermata are as a rule of small size 

 and are measured by inches. The vermiform 

 Holothurians are sometimes greatly elongated, some 

 species of Synapta measuring five feet or more in 

 length, while the extinct Crinoids had stems up to 

 seventy feet long. 



Among the heterogeneous and in many respects 

 unnatural assemblage of animals grouped together 

 by zoologists under the name Vermes, the average 

 dimensions are distinctly small. Turbellarian 

 worms, perhaps the most primitive members of the 

 group, average less than an inch in length, though 

 some forms may measure four to six inches. 

 Trematodes are similarly small ; and the great 



