64 



THE MOST PRIMITIVE METAZOA 



the gut (Fig. 22). Many genera related to Haliphysema were dis- 

 covered and they all showed certain resemblances to sponges, i.e. 

 a central cavity lined with flagellated cells, and an outer layer of 

 cells covered by a skeleton which in this case differed from that 

 of a sponge in that it was not secreted but picked up from the 

 substratum. These animals assumed such tremendous importance 

 to Haeckel that at one time he derived all the Metazoa from the 

 Physemaria, but at a later date he decided that the Physemaria 

 were on a side line from the main Gastrula. 



(b) Transverse section 



(a) Longitudinal view 



(c) Isolated choanoflagellate cell 



Fig. 22. Haeckel's view of the structure of Haliphysema. He 

 thought that the structure was that of a simple sponge, (a) shows 

 the whole animal with part of the body cut away to demonstrate 

 the inner structure, (b) is a transverse section of Haliphysema and 

 shows the gonads migrating into the interior of the animal, (c) shows 

 in more detail the structure of the inner flagellate cells. It is 

 doubtful if such a detailed pattern exists in Haliphysema or if such a 

 simple condition exists in any sponge. 



