

EMBRYOGENIC METHOD 49 



lution. The most primitive group is that of the 

 Sea star-fish or Aster ida, whose rays are not yet fixed 

 at the number of five. Haeckel considers these 

 animals as an assemblage of articulated worms 

 developed by a radiated sprouting round a central 

 worm ; but it is now known that this idea rests on 

 palaeontological observations recognized as erroneous. 

 Asterida should be derived from Crinoidea or Sea 

 lilies, through the fixation of an asterid by means 

 of a more or less lengthened stem ; some Crinoidea, 

 such as the Comatula, pass through a second stage, 

 in which they end by breaking away from their 

 stem. 



In the other two types of Echinoderms, the Urchins 

 and Holothurice, there are no. longer any free arms ; 

 these are fixed in a disc, globular in the first named, 

 prolonged in the second. This phylogeny of Haeckel 

 fairly agrees with the palseontological history of the 

 group ; yet, in the present state of our knowledge, 

 Crinoidea are quite as old as Asterida, and are known 

 to occur ever since the Cambrian age. The Echinida 

 and the Holothurise are more recent and hardly 

 reveal themselves before the second half of Primary 

 times. 



The Arthropods or articulated animals comprise 

 both aquatic types or Crustacea and terrestrial types 

 including the Insects, Myriapods, and Spiders. 

 All the first pass through a larval stage, known as 

 Nauplius, characterized by rudimentary segmenta- 

 tion and derived, according to Haeckel, from a 

 branch of the articulated worms. But this descent 

 is quite hypothetical, since the most ancient fossil- 

 bearing strata of the globe already contain true 



