116 



ZOOLOGY 



SECT. 



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. — Symn gelatinosn m 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 

 type of Sponge-structure is that 

 of the so-called Ascctia or Olynthas 

 (Fig. 85). This is not a mature form 

 - — no adult Sponge retaining such 

 simplicity of structure. 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 structure of its 

 wall in Ascetta is extremely simple. 

 Regularly arranged over the surface 

 are a number of small rounded 

 apertures, the inhalant pores; but, 

 sinoe the wall of the Sponge is very thin, these apertures 

 lead directly into the central or paragastric cavity (Fig. 86 A), 

 the long passages or canals through which the communica- 

 tion 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 thin layer of flat cells; the paragastric cavity is lined throughout 

 by choanocytes similar to those of the flagellate canals of Sycon. 



Fio. 85.— Olynthus stage of a simple 

 calcareous Sponge (Clalhrina). A 

 portion of the wall of the vase-like 

 sponge removed to show the para- 

 gastric cavity. (After Haeckel.) 



