46 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



or monophyletic origin of seven principal types of 

 this kingdom. All animals pass, in the course of 

 their embryogenic development, through the fol- 

 lowing stages : 



1. Cells without nucleus (impregnated egg), corre- 

 sponding to the type Moneron. 



2. Cell with nucleus, represented by the type 

 Amoeba. 



3. Simple polycellular state or Morula, fixed at 

 the present day under the form Synamoeba. 



4. Cellular flattened mass, or Planula. 



5. Cellular mass with cavity, or type Gastrula. 

 This stage is met with in the ontogeny of all types, 

 from the Sponges, Medusse, Corals, Worms, Tunicata, 

 and Radiata up to the Molluscs and even to the lower 

 vertebrates. It must have existed at the Laurentian 

 epoch in the shape of an hypothetical animal group, 

 the Gastreades. 



From this common starting point the evolution 

 of the six higher zoological groups follows a diver- 

 gent course. The Gastreades first form two branches. 

 In the first the animals attach themselves to the 

 sea-floor and become the root-form of the Zoophytes, 

 which are subdivided into Sponges, Polyps, and 

 Medusae. The second branch retains free power of 

 locomotion and passes into the primitive type of the 

 Worms. It is from the four sections of this last 

 group that are derived the two highest animal 

 types on the one hand the Echinoderms and the 

 Arthropods, and on the other the Molluscs and the 

 Vertebrates. 



It does not form part of the plan of this work to 

 follow out in all their details the ideas of Haeckel 



