64 BIOLOGY 



so it forms a heart exactly the same as that of the 

 higher types. 



A second example of a slightly different kind is seen 

 in the development of Penaeus. From the egg a simple 

 form called a Nauplius is produced, and this is simpler 

 than any known crustacean. This grows and moults 

 and gives rise to a second form which is called a Zoea, 

 grows and moults again and the Zoea becomes a Mysis, 

 which in its turn grows and moults and finally forms an 

 adult Penseus. Now this life-history of Penseus is prac- 

 tically unintelligible, unless we believe that Penseus does 

 to a certain extent recapitulate the stages in the history 

 of the race to which it belongs. 



There is a third generalisation which is also important, 

 namely the germ-layer theory. We saw that the egg of 

 most forms gave rise ultimately to a form in which 

 there were three distinct layers. For a long time it 

 was held that organs which were similar from the 

 anatomical standpoint must have had their origin in 

 similar layers ; this is, that homologous organs were also 

 homodermic organs. But many facts have arisen which 

 tend to shake this belief, and it is recognised more and 

 more that this germ-layer theory is not only inadequate 

 but misleading, and that the primary layers of the 

 gastrula are not strictly homologous throughout the 

 animal kingdom. 



