88 SYNOPTIC COLLECTION. 



The flagellate and collared cells of the endoderm are 

 unique, and may indicate genetic relationship with the 

 Flagellata of the Protozoa or parallelism of development 

 in two different groups. 



When fixation takes place, the sponge settles with its 

 mouth downward, after which the cloaca breaks through 

 the ectoderm, proving thereby that this opening is not a 

 primitive but a secondary character. 



The primitive, adult, ancestral form of the group of 

 sponges was a simple, skeletonless, tubular organism with 

 a water system consisting of transient pores, and a central 

 cavity with no canals and no ampullaceous sacs. 



This primitive form is inherited with certain modifica- 

 tions by many of the simpler members of the different 

 orders of sponges. By a differentiation of this primitive 

 form the most specialized sponges with canals and sacs 

 have arisen. 



The calcareous and silicious sponges are considered 

 the most generalized and the keratose sponges the most 

 specialized for the following reasons. 



The calcareous sponges, as a group, are most rudimen- 

 tary in structure. The Silicea v are found in ancient geo- 

 logical formations and in the deep seas of to-day, while 

 the Keratosa do not occur as fossils. 



The history of the development of the transitional 

 forms, the silica-and-keratose sponges, proves that the 

 silica appears first and afterward the spongin is devel- 

 oped. 



