76 THE MOST PRIMITIVE METAZOA 



mesozoan larva, but as yet no miracidium has been described which 

 is as simple as the mesozoan larva. The mesozoan larva has a very 

 short life and so it might not have time to develop the flame cells 

 found in the miracidium. Perhaps some experimental studies on 

 miracidia and the condition under which they can be maintained 

 will allow us to come to a greater understanding of the Mesozoa. 

 There is also a great deal to be discovered about the life history and 

 habits of this group of animals before we can come to any con- 

 clusion about their phylogenetic position. 



(3) The Coelenterata 



Over the early years there was a controversy over the animal 

 nature of the Coelenterata and it took some time before their true 

 animal nature was recognised (Johnstone 1838). Even so, various 

 groups of animals such as the Ectoprocta, Endoprocta and the 

 Coelenterata were grouped together mainly on the similarities of 

 their external form. 



T. H. Huxley in 1849 presented a memoir on the anatomy and 

 affinities of the Medusae to the Royal Society of London. In this 

 memoir he described how the Medusae differed from the rest of the 

 animal kingdom in that they could be regarded as having only two 

 layers whilst the other metazoans had three layers. Huxley sug- 

 gested that the layers in the Coelenterata were homologous with 

 those of the Vertebrata and that in fact the Coelenterata were 

 diploblastic. 



In Huxley's text A Manual of the Anatomy of the Inzertebrated 

 Animals published in 1891 he modified his views a little. He 

 classified the coelenterates into two main groups, the Hydrozoa 

 and the Actinozoa, and he included the Medusae in the Hvdrozoa. 



(1) Hydrozoa: (a) Hydrophora Tubularia. 



(b) Discophora Aurelia. 



(c) Siphonophora Physalia. 



(2) Actinozoa: (a) Coralligena Actinia. 



(b) Ctenophora Pleurobrachia. 



He compared the coelenterate body to a sac. " The walls of the 

 sac are composed of two cellular membranes, the outer of which is 

 termed the ectoderm, and the inner the endoderm, the former 



