xxv SIGNIFICANCE OF DEVELOPMENTAL STAGES 303 



It is important to notice that this comparatively complex 

 animal is in one stage of its existence the oosperm as 

 simple as an Amoeba ; in another the polyplast it is com- 

 parable to a Pandorina, and in a third the blastula to a 

 Volvox ; in a fourth the gastrula it corresponds in general 

 features with a Hydra ; while in a fifth the trochosphere 

 it resembles in many respects a Medusa. As in other cases 

 we have met. with, the comparatively highly-organised form 

 passes through stages in the course of its individual develop- 

 ment similar in general characters to those which, on the 

 theory of evolution, its ancestors may be considered to have 

 passed through in their gradual ascent from a lower to a 

 higher stage of organization. 



The rest of the development of Polygordius may be 

 summarized very briefly. The trunk grows so much faster 

 than the head (pro-pltts peri-stomium) that the latter under- 

 goes a relative diminution in size, finally becoming of equal 

 diameter with the trunk, as in the adult. The ciliated rings 

 are lost, the tentacles grow to their full size, the eye-spots 

 atrophy, and thus the adult form is assumed. 



