250 HIGH SCHOOL ZOOLOGY. 



(Spongidce) of very many marine families, and in fact from com- 

 paratively few species of it. The skeleton in other families is 

 either formed of calcareous spicules (Calcarea), or of siliceous and 

 horny material, together or separate (Non-calcarea) ; it is only 

 in one family of the latter that theie is no skeleton. 



8. The name Porifera is derived from the presence of 

 numerous smaller and larger apertures on the surface of the 

 living sponge, respectively called " pores " and " oscula ;" water, 

 carrying particles of food, is caused to stream through a more 

 or less complicated system of coelenteric canals, of which the 

 pores are the inhalent, and the oscula the exhalent apertures. 

 Certain parts of the canals — the ciliated chambers — are lined 

 by tall ciliated cells of characteristic shape — " collar-cells" (§ IG) 

 — and it is especially those which ai-e active in bringing about 

 the current. They are equivalent to the entoderm of the 

 Ccelenterates, the rest of the canal-system and the outer surface 

 being clad with flattened ectodermal cells, Avhile the soft, gela- 

 tinous, connective-tissue between these layers, which constitutes 

 the bulk of the "flesh" of the sponge, belongs to the mesoderm. 



In some cases the ccelentei'on, or gastrovascular cavity, has 

 only a single " osculum" (Fig. 187), and then the sponge may 

 be compared to a single Coelenterate polyp, but generally it is a 

 more complicated canal-system with numerous oscula, and the 

 resemblance to a Coelenterate colony is lessened by the absence 

 of symmetry and of constancy in the form of the body and the 

 position of the oscula. 



9. Sponges are only possessed of locomotive powers in their 

 early larval stages ; they afterwards attach themselves, and 

 possess, therefore, but little muscular tissue. Special sensitive 

 cells are present in the skin, but there are no sense-organs, such 

 as are found in the higher Ccelenterates. In addition to the 

 formation of new Individuals from eggs, a separation of buds, 



