THE GASTRiEA THEORY AND ITS SUCCESSORS. 8/ 



great extent, useless, and are therefore invaginated, 

 the nutritive granular cells being thus able to expose 

 their full surface for the acquisition of food particles. 

 The ciliated cells are enabled to carry on their respira- 

 tory function by the formation of an osculum and pores. 



According to this idea the nutritive function ought 

 to reside in the ectodermal cells of the Sponge, and 

 in the fiat cells lininsf the walls of the canals which 

 Balfour took to be derived by invagination from the 

 ectoderm, and the collared cells of the ciliated cham- 

 bers should be purely respiratory, and Metschnikoff's re- 

 searches appeared to confirm this view to a large extent. 

 The recent extensive observations of von Lendenfeld ^ 

 are, however, in direct opposition to it, demonstrating 

 that it is the collared cells which are ingestive, thus 

 confirming the earlier statements of Carter, In addition 

 to this, the fact that the development of Sycandra cannot 

 be regarded as primitive, and therefore as throwing light 

 on the ancestry of the Metazoa, indicates that the AinpJii- 

 blastiila theory is not founded on a secure basis. 



The speculations of Biitschli, which led to what that 

 author has denominated the Plakiila theory,^ had their 

 starting point in a study of an existing flagellate, 

 Goniiim, which consists of a single-layered plate of 

 cells. The transverse division of all the individuals of 

 such a colony would result in the formation of a two- 

 layered plate which Biitschli terms the Plakiila. At 

 first in all probability there would be no difference in 



1 R. von Lendenfeld. Experimentelle Untersuchungen iiber die Physi- 

 ologic der Spongien. Zeit. fur wiss. Zool. Bd. xlviii. 1889. 



2 O. Biitschli. Bemerkungen zur Gastrsea Theorie. Morph, Jahrb. 

 Bd. ix. 1884. 



