ultimately giving rise to very different tissues and structures, 

 though the generative products may be formed from any one 

 of them. 



Apparently from the very first the most primitive animals, 

 arising from two distinct protozoan prototypes, diverged in 

 three entirely different lines betvi^een which no intergradation 

 can with any degree of certainty be shown ; one of these 

 lines, represented by the sponges, shows no trace whatever 

 of further development, though it exhibits enormous interphy- 

 letic differentiation ; the second, from an entirely different 

 stock, represented by the cœlenterates, gave rise to a single 

 offshoot, the ctenophores, but developed no farther ; while 

 from the third line, represented by the flatworms and related 

 types, derived from the same stock as the first, all of the higher 

 animals have originated. 



Sponges develop from a solid morula, more or less com- 

 parable to a phanerogam embryo, or from an invaginate 

 gastrula which attaches itself by the blastopore, developing 

 directly and at once into an amorphous mass consisting of 

 a community of cells imperfectly integrated and showing 

 relatively little division of labor or unified life with no definite 

 organs or tissues, masses which to all intents and purposes 

 are colonies of more or less differentiated and localized 

 individual and independent cells, in spite of their origin 

 comparable to colonies of infusorians rather than to the bodies 

 of the higher animals. 



The sponge mass develops radially about the original axis 

 of the embryo as a center into a more or less subcircular, 

 subradiate, cup-like, foliaceous or digitiform body as a result 

 of the efforts of the sponge to expose as much of its surface 

 as possible to the water in order to insure the maximum food 

 supply. The development of the original unit is always radial 

 and that of the mass fundamentally radial, while in the thal- 

 lophytes the exactly similar forms, occuring especially in the 

 lichens, resulting from their efforts to expose as much of their 

 substance as possible to the air, are derived originally through 

 linear development by masses of elongate or linear cells. 



The marked discontinuity between the sponges and all 

 other animals with the possible exception of certain infusorians 

 indicates that they form an isolated group entirely distinct 



