72 PRIMITIVE ANIMALS 



in the Trochosphere, as the two anterior pairs of 

 pouches are transformed into a system of fluid-con- 

 taining vessels which function as a kind of circulatory 

 system. The posterior pair of pouches are, however, 

 transformed into the reproductive organs, and in this 

 respect they agree with the coelom of all coelomate 

 animals, in which the reproductive organs are in- 

 variably formed from the coelom. 



Another point of difference between the Trocho- 

 sphere and the Auricularia is found in the constant 

 absence of an excretory nephridium in the latter. 



The Auricularia is a bilaterally symmetrical 

 organism, but in its transformation into the adult 

 form of Echinoderm it becomes peculiarly twisted 

 and asymmetrical at first, and then a new type of 

 symmetry, the radial, is imposed upon it, producing 

 the peculiar forms of the Star-fishes and Urchins and 

 Sea-lilies with which everyone is familiar. 



Now it is a most noteworthy fact that an animal 

 differing entirely in the adult state from the Echino- 

 derms, viz. Balanoglossus (a worm-like burrowing 

 creature which is regarded as being most nearly 

 related to the Vertebrata, owing to its possession of 

 such characteristic Vertebrate organs as gill-slits and 

 of certain other structures), possesses a free-swimming 

 transparent larval form, the Tornaria, which is practi- 

 cally identical in all its features with the Auricularian 

 larva of Echinoderms. There is absolutely nothing 



