146 AN INTRODUCTION TO ZOOLOGY. 



preted in the way already named, viz., as representing 

 the three usual body-layers, yet the different forms of 

 larval development that have been observed are diffi- 

 cult to reconcile with each other, and much remains 

 to be learnt about the sponges. They were formerly 

 supposed to have no structure beyond what has been 

 stated; but cells have been described in some kinds 

 of sponge, which have been interpreted as rudimen- 

 tary sense-cells. 



The Sponges have been classified according to the 

 nature of their skeletons . one group includes sponges 

 with calcareous skeleton, the others, respectivelj', 

 sponges with no skeleton, with horny skeletons, 

 silicious skeletons, and skeletons mixed of both. 

 These skeletons usually consist chiefly of isolated 

 spicules (needles), which present very curious and 

 beautiful shapes, and are favourite objects for the 

 microscope (fig. 29). 



Sponges may be abundantly found on rocks and 

 bits of seaweed near low tidemark on our coasts, 

 though the large species that we buy in the dry state 

 are the growth of warmer seas. They are brilliantly 

 coloured, orange and olive-green being among the 

 commonest kinds. There is also a green fresh-water 

 sponge, Spongilla, which is found in English streams. 



The Cnidaeia are divided into two classes, the 

 corals, Anthozoa (flower-animals) or Actinozoa (ray- 

 animals), and the jelly-fishes, Hydkozoa, or Polypome- 

 DVSM. Besides these, another division, the Ctenophoea, 

 is sometimes made into a separate class. 



