MORPHOLOGY OF ANIMALS 



ONTOGENETIC SERIES 



1. Egg 



2. Cleavage - 



3. Blastula - 



4. Radial Gastrula 



TAXONOMIC SERIES 



Protozoan 

 Protozoan Colony 

 Volvox-like Forms 

 Simple Sponge, Hydroid 



5. Bilateral Gastrula Polyclad-like Forms 



/mmmmfmminm mmmrm 

 A B 



Fig. 2. Diagrams of Protaxonia in which the gastrula axis becomes the chief axis of the 

 adult. A, a simple sponge in which the gastrula becomes attached by its oral pole, the 

 blastopore then closes and a new opening into the gastric cavity, the osculum {os), appears 

 at the apical pole ; B, a Hydroid which has attached by the apical pole of the gastrula 

 while the blastopore becomes the mouth {m) which is then surrounded by a row of tentacles ; 

 C, a ctenophore or jelly-sphere which swims by means of plates of fused cilia (8 plates are 

 shown) ; the apical pole of the gastrula gives rise to the sense organ (jo) of the adult while 

 the blastopore becomes the mouth {m). (After Hatschek). 



II. MODIFICATIONS AND COMPLICATIONS 

 OF THE GASTRULA 



1. Axes and Symmetry 



(a) Protaxonia are the lowest Metazoa (Spongiaria, 

 Cnidaria, Ctenophora) in which the chief axis of the gas- 

 trula becomes the chief axis of the adult (Fig. 2). In the 



1: 15: 



