METAZOA PORIFERA. 77 



being more symmetrical and constant in form than the 

 members of other orders of Silicea. They also stand 

 correspondingly near to the Calcarea in their organiza- 

 tion, as has already been stated. The mesoderm is not 

 so thin, but it approximates to the condition of that of the 

 more primitive calcareous sponges, and in accord with 

 this the excurrent canals are not found, and the ampullae 

 open directly into the cloacal trunk in some forms. Thus 

 the organization is just a grade more specialized than in 

 the Ascones with their digestive cells in the central trunk, 

 and less specialized than Sycones with their ampullae in 

 the canals of the lateral branches of the water system. 



SILICEA. LITHISTIDAE. 



These forms have a thick stony wall and irregular 

 spicules some of which are cleft into ragged branches. 

 The fossil Tragos (No. 71) is a representative. It is 

 shaped like a funnel and the exterior wall is often wrinkled 

 concentrically. 



SILICEA. TETRACTINELLIDA. 



Tetilla sandalina Sollas (PI. 72, fig. i) is a representa- 

 tive of the simple Tetractinellida. It has a single open- 

 ing at one end with papillae at the other. The outer 

 portion of the sponge is soft, not differing essentially 

 from the inner. The mesoderm is slightly developed. 

 The ampullaceous sacs (fig. 2) with their flagellate cells 

 are large, and open by a wide mouth into the excurrent 

 canals. The spicules vary from a straight rod to an 

 S-shaped form (fig. 3). They are seen in fig. 2, where 

 the straight ones overlap, forming "spicular fibers." 



Tethya (No. 73; PL 74, figs, i, 2) has a spherical 

 form with one or more small openings. The outer sur- 



