434 NATURAL HISTORY AND ANATOMY OF THALASSEMIA, ETC. 



From the anatomical description we have now given, it is 

 evident that the genera Echiurus and Thalassema must be 

 arranged in the class Ecliinoclcrmata. The body filled with 

 sea-water — the respiratory apparatus — the digestive system — 

 and the intestinal venous trunk, are the leading anatomical 

 peculiarities, and are characteristic of the echinodermatous 

 auimals. The colour and circulation of the blood, the want 

 of an aquiferous system, the ventral nervous cord, and the 

 muscular system, show the relation of these animals to the 

 Annclides, and prove that the transition from a vermiform 

 radiate animal to a true articulate animal, is effected by the 

 symmetrical atrophy and hypertrophy of certain of the radiate 

 elements in each ring. 



