SECT. III. 



CALCAREOUS AXD SILICEOUS. 



59 



which consists of a network of tubular, horny, tough, 

 and elastic fibres which cross in every direction. They 

 have no spicules or very- 

 few ; and when such do 

 project from the horny 

 skeleton, they are gene- 

 rally conical, attached 

 by their bases, and their 

 surface is often beset 

 with little spines ar- 

 ranged at regular inter- 

 vals, which gives them 

 a jointed appearance. 

 The common sponge 

 which is so abundant in 

 the Mediterranean has ** 103 ' ^ on ofSponge ' 



many forms ; those from the coast of North America 

 are no less varied, but that most used in the United 

 States is turbinate, concave, soft, and tomentose. 



In the calcareous sponges a mass of three-rayed spi- 

 cules surround the interior canals, where they are held 

 together by a cartilaginous substance which is wanting 

 in the horny sponges, but which remains in this order 

 after the destruction of the more delicate matter when 

 the sponge is dried. 3 The pores are also occasionally 

 defended by the projecting points of half buried spines. 



In nearly every species of this order the pores on the 

 surface are protected by spicules ; and they are also pro- 

 jected from the surface of the large cloacal cavity, and 

 curved towards its opening, to defend it from Annelids 

 and other enemies. 4 Some species have a long ciliary 

 fringe at the orifice of the cavity, through which the 

 water may pass out, but no animal can come in. 



8 ' Histoire Natnrelle des Animaux sans Vertebras ' par MM. Deshayes et 

 H. Milne-Edwards. 



4 Memoir by Dr. Bowerbank in the Transactions of the Microscopic 

 Society. 



