20 



EVOLUTION AND GENETICS 



Ectoderm / 

 Entoderm 

 Coelenteron 



Perlsarc 



r.iL— Blastostyle 



species as those mentioned are easily recognized and sharply de- 

 limited, a closer scrutiny of individual characteristics shows that 



in many cases it is ex- 

 tremely difficult to say 

 exactly what characters 

 define the species and 

 how they are separated 

 from each other. We 

 have never been at loss 

 for examples of species, 

 yet there has never 

 been unanimity of 

 opinion among scien- 

 tists as to what species 

 really are. 



The belief has been 

 -Gonotheca pxpresscd that there is 

 '— Meuusa-buj no real group in nature, 

 but that only individ- 

 uals are real entities 

 and the groups into 

 which we gather them 

 for our scientific rec- 

 ords nothing more 

 than conveniences. In 

 contrast the other ex- 

 treme has been urged 

 from time to time, that 

 as some individuals re- 

 semble each other more 

 than they do any other 

 organisms, they must 

 constitute a natural 

 group with definite 

 limits, even though 

 they may vary within 

 these limits. As is us- 

 ually the case, the opinion has gradually developed that there 

 is sound value in both interpretations. It is now supposed 

 that there are such things as natural groups which may aptly 



;^^...Statocyst 



Radial canals 



^,. Reproductive organs 



Tentacles 



Mouth 



Fig. 4. — Obelin. A, stalk bearing hydranths; B, 

 medusoid. (From Parker and Haswell.) 



