576 SEMIVERTEBRATES. 



appear to have been derived by a series of changes mainly correlated with the 

 assumption of their characteristic radial symmetry ; while the Chordates retained 

 the original bilateral symmetry, at the same time undergoing certain changes, into 

 the consideration of which it will be unnecessary to enter in this place. Still more 

 complicated are the changes necessary to evolve Balanoglossus and its allies from 

 the ancestral form. Such of our readers as are desirous of pursuing further this 

 interesting subject, may be referred to the works of the observer last mentioned. 



A PYROSOMA- COLONY. 



In life the colony assumes a horizontal position. 



