198 THE DOCTRINE OF DESCENT. 



body. We will not describe the highly complex trans- 

 formations of the larva here into an ophiura, there into 

 a sea-urchin, and here again into a sea-cucumber; but we 

 will only inquire what can be the cause of this accord- 

 ance in the earlier stages of individual development. 

 There is no reasonable answer but the derivation of all 

 echinoderms known to us from an older form, in the 

 development of which our larva likewise appeared, and 

 from which this common phase of development was 

 transmitted to the whole family. But it is allowable 

 to ask, further, how from a bilateral larva, one, that is, 

 symmetric on two sides, should be evolved in animals 

 of radiate structure, as are the greater number of mature 

 echinoderms? 



On this point Haeckel instituted a conjecture which 

 at first exasperated the systematizers of the old school, 

 but which now gains more and more footing, and is 

 supported by the most recent comparative investigations, 

 such as those of Hoffmann " On the Minute Anatomy 

 of the Starfish " (" Ueber die feinere Anatomie der See- 

 sterne "). The boat-shaped larva of the Echinoderms, 

 especially a modification occurring in the star-fish, strik- 

 ingly resembles a certain larval type of the marine 

 Annelida. And as in the structure and distribution of 

 the parts of the rays of the echinoderms, especially of 

 the star-fish, an unmistakable resemblance with the rela- 

 tive distribution and succession of parts of the 

 Annelids is observable, Haeckel regards the Echinoderms 

 as an offshoot of the Annelids. He considers that the 

 oldest, and to us unknown, echinoderms originated as 

 annelid stems; the anterior end of the bilateral annulose 

 parent-animal budding out gemmules in a radiate ar- 



