., 



PHYLUM AND CLASS PORIFERA 



107 



ment like the ribs of an open fan. Often the lamella becomes 

 folded, and sometimes there is a coalescence between the folds, 

 resulting in the development of a honey-comb-like form of 

 sponge. 



Sponges resemble plants, and differ from the higher groups of 

 animals, in the readiness with which, in many cases, their form 

 becomes modified during growth by 

 external conditions (environment). 

 Different individuals of the same 

 kind of Sponge, while still exhibiting 

 the same essential structure and the 

 same general mode of growth, may 

 present a variety of minor differences 

 of form, in accordance with differ- 

 ences in the form of the supporting 

 surface or in the action of waves and 

 currents. 



Leading Modifications of 

 Structure. Sycon gelatinosum be- 

 longs to a type of Sponges interme- 

 diate between the very simplest forms 

 on the one hand, and the more com- 

 plex on the other. The simplest and 

 most primitive of known Sponges is 

 one named Ascetta primordialis (Fig. 

 76). It is vase-shaped, contracted at 

 the base to form a sort of stalk by 

 the expanded extremity of which it 

 is attached ; at the opposite or free 

 end is the circular osculum. So far 

 there is a considerable resemblance 

 to Sycon gelatinosum ; but the struc- 

 ture of its wall in Ascetta is ex- 

 tremely simple. Regularly arranged 

 over the surface are a number of 

 small rounded apertures, the inhalant 

 or incurrent pores ; but, since the 

 wall of the Sponge is very thin, these 

 apertures lead directly into the cen- 

 tral or paragastric cavity (Fig. 77, A), 

 the long passages or canals through which the communication is 

 effected in Sycon being absent. The wall consists of the same 

 three layers as in Sycon, but the middle .one, though it contains 

 a small number of spicules, is very thin. The ectoderm is a syn- 

 cytium ; the endoderm, which lines the paragastric cavity, consists 

 throughout of flagellate collared cells similar to those of the fla- 

 gellate canals of Sycon. 



FIG. 76. Ascetta primordialis. 



A portion of the wall of the vase- 

 like sponge removed to show 

 the paragastric cavity. (After 

 Haecl " 



