68 



THE MOST PRIMITIVE METAZOA 



in the Hydrozoa is formed by the wandering in of cells, in the 

 Scyphozoa, Anthozoa and higher animals the endoderm is often 

 formed by invagination. Since on other grounds Jagersten thinks 

 that the Anthozoa are more primitive than the Hydrozoa, he 

 suggests that invagination is the more primitive system in the 

 formation of endoderm and that the primitive larva had a hollow 

 centre, i.e. a gut, and was not solid as has been suggested by 

 Hyman (1940). 



The following system is therefore suggested by Jagersten to 

 explain the development of the Porifera from the bilaterogastrea. 

 The bilaterogastrea settled on the sea floor and placed the 

 middle of its elongated mouth on the substratum. The water and 

 food material flowed in through the mouth and out via the anus. 

 The mouth later became folded and developed a series of pores as 

 shown in Fig. 24. The anus remained a single structure and 

 migrated to a dorsal position to become the exhalant opening. The 

 animal then had the form of a sponge though it would still have 

 to develop the peculiar histological structure of the Porifera. 

 J agersten's view is of interest in showing what could have happened. 

 Whether the Porifera did actually arise in this manner is open to 

 doubt. 



Fig. 24. Jagersten's view of the evolution of the sponges. The 

 bilaterogastrea settled on the bottom (a), raised its mouth from 

 the substratum (b) and then divided the mouth into many oscula 

 (c). The anus then migrated dorsally (f, g). (e) is a transverse 



longitudinal section of a. 



